PX4/NuttX branch test: master

NuttX:
 - https://github.com/PX4-NuttX/nuttx/tree/master
 - PX4/NuttX@15f2889

Changes from PX4/NuttX (PX4/NuttX@9331fda) in current PX4/master (e4a526e)
PX4/NuttX@9331fda...15f2889

15f2889 (HEAD, origin/master) Merged in masayuki2009/nuttx.nuttx/fix_fe310_signal (pull request #1099)
d80d6b8 Merged in raiden00/nuttx_nrf52 (pull request #1098)
466ab56 Merged in masayuki2009/nuttx.nuttx/fe310_gpio (pull request #1097)
13038d0 Documentation/NuttX.html:  Fix MIPS32 references.
fed50a1 include/nuttx/wireless/wireless.h: Correct number of network commands.
e7c9c89 Trivial updates to files modified in last PR based on results of tools/nxstyle.
a0429bc Merged in raiden00/nuttx_nrf52 (pull request #1096)
bc7566a arch/arm/include/armv7-m/syscall.h:  ARM EABI specifies that the stack should be aligned by 8 on function calls, inside the function is not required to be aligned by 8. Since these functions call svc, compiler doesn't know that the svc is a function, therefore it does not do any stack management. This change pushes an even number of args to the stack and maintains an 8 byte alignment. I've checked the assembly and it doesn't cause any more overhead that the hand written assembly.
68b0ba8 arch/sim/src/sim/up_framebuffer.c:  Replace all _info/_err with ginfo/gerr.
3a5e2b7 arch/sim/src/:  Simulator restore the console even error handler call host exit directly.

Apps:
 - https://github.com/PX4/NuttX-apps/tree/master
 - PX4/NuttX-apps@2a462c7

Changes from PX4/NuttX-apps (PX4/NuttX-apps@91b6ad6) in current PX4/master (e4a526e)
PX4/NuttX-apps@91b6ad6...2a462c7

2a462c78 (HEAD -> master, origin/master) nshlib/nsh_parse.c:  Replacement implementation for 9defae8af641752506d92b72ea68c8f94d24d580.  This addes conditional compilation on the definition of g_nullstring[] to avoid the warning.  Suggested by SUZUKI Y <NBG03015@nifty.ne.jp>
6bfd6166 Revert "apps/nshlib/nsh_parse.c:  Fix warning: 'g_nullstring defined but not used'.  Use  directly since the usage is triggered by a complex Kconfig combination."
9b77f807 apps/netutils/dhcpd/dhcpd.c:  Correct backward conditional logic in dhcpd_leaseexpired().  Noted by surya prakash <suryaec1684@gmail.com>.
9defae8a apps/nshlib/nsh_parse.c:  Fix warning: 'g_nullstring defined but not used'.  Use  directly since the usage is triggered by a complex Kconfig combination.
3a2bd2c0 apps/netutils/ntpclient/ntpclient.c: Merge local structure variables xmit and recv into pkt to save the stack.
e48a74f3 apps/Application.mk: Convert object names to guarantee uniqueness.
5ec8fc94 apps/nshlib/nsh_fileapps.c:  nsh_fileapp() should return 1 if the application task was spawned successfully but returned failure exit status.  For example, nsh shouldn't output "/bin/ping: command not found":
77a3b083 apps/nshlib/nsh_parse.c:  Ensure /bin/ping and ping work at the same time.  Don't skip nsh_builtin even CONFIG_NSH_FILE_APPS or CONFIG_FS_BINFS equal y.
3da8091d Include nuttx/symtab.h instead of nuttx/binfmt/symtab.h since symtab.h under binfmt folder is for internal use.
7479cd68 apps/system/nsh/nsh_main.c:  Remove HAVE_DUMMY_SYMTAB logic since BINFS can run without it.