tools/boot/install-boot.sh was assuming that if a device was passed in,
it should operate on the current system and run efibootmgr etc. to
update the boot manager. However, rootgen.sh passes a md(4) device and
not a fixed disk.
Add a -n option to install-boot.sh to tell it to operate in 'offline'
mode and not try to update the current system.
Also, source install-boot.sh in rootgen.sh to allow it to find and
call make_esp_file etc.