These FMU and FMUv2 bootloader images are built from:

  https://github.com/PX4/Bootloader

They are in ROMFS to make it easier for users to update their
bootloaders using:

 bl_update /etc/bootloader/fmu_bl.bin

from a nsh prompt. Users can get a nsh prompt either via the CLI in
test -> shell, or by booting with no SD card installed