Rename CONFIG_AUDIO_DEVICES to CONFIG_DRIVERS_AUDIO to conform better to the evolving configuration naming standard.

examples/fb:  Add a simple test for the framebuffer character driver.  This initial commit just verfies some interfaces and initialization logic.  Still need to add to graphics update logic.

examples/adc: Fix g_adcstate.count initialization.

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

examples: add DAC example

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

apps/examples/nsh: Remove APPNAME, PRIORITY, and STACKSIZE settings from Makefile to avoid showing nsh in Builtin Apps.

dhcpd: Prevent buffer overflow in dhcpd_addoption.  offset represents distance from start of option buffer.  So this should be changed current option pointer minus start of buffer.

NSH:  Add conditions so that ifconfig and ifup will not be disabled if we are using only PF_IEEE802154

commit 6fbbca4ce308f5d8130d795342c7639555a7e5bf
Author: Gregory Nutt <gnutt@nuttx.org>
Date:   Sat Aug 19 08:46:27 2017 -0600

    apps/examples/pf_ieee802154:  Add PANID to command line options; Cannot bind to address zero... There is no counterpart to INADDR_ANY for these radios (not now at least).

commit 354111108a88f549248537e79dbf79998f458ded
Author: Gregory Nutt <gnutt@nuttx.org>
Date:   Fri Aug 18 17:47:10 2017 -0600

    PF_IEEE802154:  A few bugfixes

commit f1bb947609ed5e9e8816234653d6c8ee24585079
Author: Gregory Nutt <gnutt@nuttx.org>
Date:   Fri Aug 18 15:17:32 2017 -0600

    Add file missed in last commit

commit 9a73930e864db83609b5cc0d7c940d8efeaade66
Merge: 315b888f c433e07a
Author: Gregory Nutt <gnutt@nuttx.org>
Date:   Fri Aug 18 12:08:03 2017 -0600

    Merge remote-tracking branch 'origin/master' into pf_ieee802154

commit 315b888f4e72523d90b526edc5a82fb97678ff7e
Author: Gregory Nutt <gnutt@nuttx.org>
Date:   Fri Aug 18 12:06:51 2017 -0600

    examples/pf_ieee802154:  Add for testing PF_IEEE802154 sockets.

The last change added .PRECIOUS: $(BIN) to everyway Makefile in hopes the libapps.a will not be deleted on Control-C (unproven so far).  However, $(BIN) may contain back slashes in some build platforms and that causes problems for make:  It can't handle the backslashes in the .PRECIOUS dependency list.  $(BIN) replaced with ../../libapp.a, where the number of ../ depends on the depth of the Makefile in the apps/ source tree.

All Makefiles:  Add .PRECIOUS: apps/libapps.a to every Makefile.  Hopefully this will end awkward problems when you Control-C out of a build and libapps.a is deleted.

Application.mk:  Add the .PRECIOUS target to prevent libapps.a from being deleted when the make is interrupted.

NSH addroute and delroute:  All expression of the netmask in IPv4 CIDR or IPv6 slash notation.  This really reduces the pain of using the commands, especially for IPv6.

NSH library:  Update addroute and delroute command.  The would support either IPv4 or IPv6, but not both.