Fix install-boot.sh and rootgen.sh UEFI bugs

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 -u option to install-boot.sh to tell it to update the system
in-place and run efibootmgr etc.

Also, source install-boot.sh in rootgen.sh to allow it to find and
call make_esp_file etc. And pass the loader file to make_esp_file instead
of a directory name.

Reported by: ian
Reviewed by: ian,imp,tsoome
Differential Revision: https://reviews.freebsd.org/D19992