Add support for MCUBoot.
Two new configurations are added:
- mcuboot-loader: mcuboot-loader app used as a bootloader.
main_mcuboot_loader as entrypoint
- mcuboot-app: used as mcuboot agent app. Needs to be
signed manually through "imgtool sign --pad --align 4 -v 0 -s
auto -H 0x200 --pad-header -S 0xc0000 nuttx.hex nuttx_sign.bin"
Signed-off-by: Andres Sanchez <tito97_sp@hotmail.com>