PX4/NuttX branch test: master

NuttX:
 - https://github.com/PX4-NuttX/nuttx/tree/master
 - PX4/NuttX@d450f5c

Changes from PX4/NuttX (PX4/NuttX@befe53e) in current PX4/master (341dd6b)
PX4/NuttX@befe53e...d450f5c

d450f5c (HEAD, origin/master) Add support for Litex VexRiscV.
9b88ecc sim: Make SIM_NETDEV_TAP depends on LINUX||WINDOWS
5d5878d NETWORK-VPNKIT.txt: Fix a typo
88d59ba arch/arm/src/imxrt/imxrt_usbdev.c: Add OUT interrupt endpoint support
6baebce arch: esp32: Fix compile error for smp
8981413 Don't check position of comments right of code when separated by preprocessor line without comment.
a427b28 Add a sample config with CONFIG_SIM_NETDEV_VPNKIT
a1539cf NETWORK-VPNKIT.txt: Mention vpnkit setup
d938511 Document CONFIG_SIM_NETDEV_VPNKIT
a5f8b20 sim: Add a netdev backed by VPNKit

Apps:
 - https://github.com/PX4/NuttX-apps/tree/master
 - PX4/NuttX-apps@b27d694

Changes from PX4/NuttX-apps (PX4/NuttX-apps@91b6ad6) in current PX4/master (341dd6b)
PX4/NuttX-apps@91b6ad6...b27d694

b27d694d (HEAD -> master, origin/master) 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
382284b4 nsh_dbgcmds.c: Appease nxstyle complaints
2b47187a nsh_command.c: Appease nxstyle complaints