Remove configurations using canutils/libuavcan and examples/uavcan.  libuavcan has not built for a year or so.  The basic problem is that as NuttX advances, the old frozen versino of libuavcan has become absolute because it violates the portable POSIX OS interface.  No one is maintaining the port so there is no alternative but to remove it.

Documentation/NuttxPortingGuide.html:  Some updated wording of description of the directories need for a complete board configuration.

i2c/i2c_master.h:   Change incorrect comment about I2C_TRANSFER return value to indicate that the method returns zero on success rather than the number of bytes transferred.

drivers/sensors/sht21:  Change I2C_TRANSFER return value so that it return on zero on success, not the count of bytes transferred.

Redo commit 7244ae9a.  The same trivial change was done with an online edit, but this resulted in some corruption of special characters in the file, likely because of differences with the character set used by the Bitbucket online editor.

This reverts commit 7244ae9a.

tools/genromfs.c:  Improve check for empty directories.  If the directory contains only unsupported, special files, then it is still empty.  Fix some collection of statistics that was cause statfs() to return some incorrect values.  Add logic to handle the case where the host and the target differ in endian-ness.  fs/procfs:  In show mountpoints, add support so that the CROMFS file system type is recognized.

fs/cromfs: More directory traversal fixes.  tools/gencromfs.c:  Add ability use a callback with directory traversal.  Not currently used but left in place in case it is needed in the future.

fs/cromfs and tools/gencromfs:  Various fixes for traversal relative file paths.  Biggest changes is in types used:  Cannot use size_t or mode_t in common structures because they have different sizes on the (64-bit) host and the (32-bit) target.  Use uint32_t instead of size_t for offsets.  Use uint16_t instead of mode_t.

fs/cromfs:  Fix read logic.  tools/genromfs.c:  Last node in directory needs to have zero for peer offset.  Still some issues with empty directories.

Squashed commit of the following:

    tools/gencromfs.c:  Seems basically functional but has not has not yet been integrated with CROMFS.
    tools/gencromfs.c:  Revist previous commit.  Let's make no assumptions about the compatibility of the host system and NuttX.
    tools/gencromfs.c:  I think access mode bits are sufficiently standard that we can just copy them from the host.  This is don't mostly to pick up the executable bits and other conbinations of bits without doing a bit-for-bit decode.
    tools/gencromfs.c:  Code complete but only partially functional.
    tools/gencromfs.c:  type definitions must precede data definitions
    tools/gencromfs.c:  Adds a little more logic to gencromfs tools.  Still not complete but can traverse directories and generate directory and hard-link nodes.
    tools/gencromfs.c: Beginning of a genromfs tool that will eventually be used for generation of CROMFS file system images.

mm/iob/iob_copyin.c:  Fixed problem with send() ret value when using nonblocking io over buffered tcp socket

Fix SMP related bugs

* sched/sched: Fix a deadlock in SMP mode

    Two months ago, I introduced sched_tasklist_lock() and
    sched_tasklist_unlock() to protect tasklists in SMP mode.
    Actually, this change works pretty well for HTTP audio
    streaming aging test with lc823450-xgevk.

    However, I found a deadlock in the scheduler when I tried
    similar aging tests with DVFS autonomous mode where CPU
    clock speed changed based on cpu load. In this case, call
    sequences were as follows;

    cpu1: sched_unlock()->sched_mergepending()->sched_addreadytorun()->up_cpu_pause()
    cpu0: sched_lock()->sched_mergepending()

    To avoid this deadlock, I added sched_tasklist_unlock() when calling
    up_cpu_pause() and sched_addreadytorun(). Also, added
    sched_tasklist_lock() after the call.

Signed-off-by: Masayuki Ishikawa <Masayuki.Ishikawa@jp.sony.com>

* libc: Add critical section in lib_filesem.c for SMP

    To set my_pid into fs_folder atomically in SMP mode,
    critical section API must be used.

Signed-off-by: Masayuki Ishikawa <Masayuki.Ishikawa@jp.sony.com>

* mm: Add critical section in mm_sem.c for SMP

    To set my_pid into mm_folder atomically in SMP mode,
    critical section API must be used.

Signed-off-by: Masayuki Ishikawa <Masayuki.Ishikawa@jp.sony.com>

* net: Add critical section in net_lock.c for SMP

    To set my pid (me) into fs_folder atomically in SMP mode,
    critical section API must be used.

Signed-off-by: Masayuki Ishikawa <Masayuki.Ishikawa@jp.sony.com>

Approved-by: Gregory Nutt <gnutt@nuttx.org>

libc/lzf:  Fix some typos in code that was in conditional logic that was not building.  Change a literal use of 13 to HLOG which used to be 13.

Squashed commit of the following:

    fs/cromfs:  Now depends on EXPERIMENTAL because it is not yet verified.
    fs/cromfs:  This commit brings the CROMFS file system to code complete.  Still completely untested.  Next steps:  Need a tool to generate CROMFS file system images and a test case under apps/
    fs/cromfs:  Add logic to traverse the in-memory file system nodes.
    fs/cromfs:  Add initial support for an in-memory, compressed, read-only file system.

configs/stm32f4discovery:  Building of stm32_appinit.c should not depend on CONFIG_NSH_LIBRARY but on CONFIG_LIB_BOARDCTL.

configs/stm32f4discover/posix_spawn:  With recent changes, needs to enabled boardctl() w/symbol table support.