AP_HAL: added hold_in_bootloader parameter to scheduler->reboot()
Rover: support reboot to bootloader
Copter: support reboot to bootloader
Plane: support reboot to bootloader
build: cope with config.mk from configure

the HAL_BOARD syntax doesn't work with shell with set -e
PX4: added FMU bootloader images to ROMFS

this makes it much easier for users to update their bootloaders
PX4: moved UART handling to its own thread

this prevents slow writes to the SD card from interfering with MAVLink
or GPS serial communication
PX4: boot the FMU bootloader in ROMFS
autotest: added comment for typical command line
Rover: raise GPS serial buffer size
AP_GPS: use the mask bits to only set dynamic model

this ensures we don't try to change any other CFG_NAV5 settings
AP_GPS: reset _fix_count to 0 on nav settings update

this makes it run every 20s exactly