release: Restore copy of boot.scr for some board

This is not a problem for 12-CURRENT as EFI boot works but it doesn't
for 11.
While here some board arm_install_uboot also copy ubldr.bin et create
firstboot files but it's already done in arm_install_boot

Reviewed by: gjb
MFC after: 3 days
Differential Revision: https://reviews.freebsd.org/D16481