PX4/NuttX branch test: master

NuttX:
 - https://github.com/PX4-NuttX/nuttx/tree/master
 - PX4/NuttX@547a3cb

Changes from PX4/NuttX (PX4/NuttX@befe53e) in current PX4/master (1e57846)
PX4/NuttX@befe53e...547a3cb

547a3cb (HEAD, origin/master) Run all .c and .h files in previous commits through nxstyle.
533528a libs/libc:  Remove support CONFIG_FS_READABLE
d22b22a syscall/:  Remove support for CONFIG_FS_READABLE
2b532ae fs/: Remove support for CONFIG_FS_READABLE
4312865 drivers/:  Remove support for CONFIG_FS_READABLE
7a871e2 fs/:  Remove support for CONFIG_FS_WRITABLE
66ab039 drivers/:  Remove support for CONFIG_FS_WRITABLE
382513e libs/libc:  Remove support for CONFIG_FS_WRITABLE
74ce4ab include/:  Remove support for CONFIG_FS_WRITABLE
9120c33 Documentation/:  Remove support for CONFIG_FS_WRITABLE

Apps:
 - https://github.com/PX4/NuttX-apps/tree/master
 - PX4/NuttX-apps@54b375d

Changes from PX4/NuttX-apps (PX4/NuttX-apps@91b6ad6) in current PX4/master (1e57846)
PX4/NuttX-apps@91b6ad6...54b375d

54b375dc (HEAD -> master, origin/master) examples: Fix elf/module/posix_spawn sometimes parallel build errors at link time
ad552c8c Makefile don't change symtab_apps.c timestamp if no any change
b27d694d example/module: update SYMTAB_SRC dependency to fix FSROOT not populated
47c0df6a Revert "examples: elf: Fix parallel build errors at link time"
d7886374 Revert "examples/posix_spawn: Fix parallel build errors at link time"
06c9f935 Makefile: remove context dependency to avoid apps context build twice
0e13730a Merge pull request #120 from liuguo09/posix_spawn_nightly_build_fix
80428f56 examples/slcd/slcd_main.c: Fix nxstyle issues.
f17d89d0 examples/slcd: fix priv->fd that was used by slcd_flush before being set
e1d9e822 nshlib: Fix a few strange whitespace in command help text