Upstream stm32f7 serial fixes

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

  1) Revert the inherited dma bug from the stm32
     see https://bitbucket.org/nuttx/nuttx/commits/df9ae3c13fc2fff2c21ebdb098c520b11f43280d
     for details.

  2) Most all CR1-CR3 settings can not be configured while UE
     is true. Threfore we make all operation atomic and disable
     UE and restore it's originalstate on exit.

net/:  Permit net/neighbor to build when IPv6 is defined, but not Ethernet.  Needs more work to support 6loWPAN.  Also included, some minor configuration updates for 6loWPAN.

User-space networking stack API allows user-space daemon to
provide TCP/IP stack implementation for NuttX network.

Main use for this is to allow use and seamless integration of
HW-provided TCP/IP stacks to NuttX.

For example, user-space daemon can translate /dev/usrsock
API requests to HW TCP/IP API requests while rest of the
user-space can access standard socket API, with socket
descriptors that can be used with NuttX system calls.

From Harri Luhtala <harri.luhtala@haltian.com>. Tested with earlier
version of NuttX; with current version checked that it compiles.

Change STM32 tickless to use only one timer

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

Entropy pool gathers environmental noise from device drivers, user-space, etc., and returns good random numbers, suitable for cryptographic use. Based on entropy pool design from *BSDs and uses BLAKE2Xs algorithm for CSPRNG output.

Patch also adds /dev/urandom support for using entropy pool RNG and new 'getrandom' system call for getting randomness without file-descriptor usage (thus avoiding file-descriptor exhaustion attacks). The 'getrandom' interface is similar as 'getentropy' and 'getrandom' available on OpenBSD and Linux respectively.

net/local: connect: Fix warning with gcc-arm-none-eabi-5-2016q1.  Using compiler from gcc-arm-none-eabi-5-2016q1 toolchain:

  gcc version 5.3.1 20160307 (release) [ARM/embedded-5-branch revision 234589] (GNU Tools for ARM Embedded Processors)

gives error:

  local/local_connect.c:188:7: error: '_local_semtake' is static but used in inline function 'local_stream_connect' which is not static [-Werror]

this is due to compiler enforcing ISO/IEC 9899:1999 6.7.4.3:  "An inline definition of a function with external linkage shall not contain a definition of a modifiable object with static storage duration, and shall not contain a reference to an identifier with internal linkage."

Fix by making inlined caller to have internal linkage as well.

6loWPAN: IEEE802.15.4 MAC driver will need a special form of the network device structure to manage fragmentation of the large packet into frames.