arm64: Turn u-boot.bin back into an ELF file after relocate-rela

While performing relocations on u-boot.bin should be good enough for
booting on real hardware, some simulators insist on booting an ELF file
(and yet don't perform ELF relocations), so convert the relocated
binary back into an ELF file.  This can go away in the future if we
change relocate-rela to operate directly on the ELF file, or if and
when we stop caring about a simulator with this restriction.

Signed-off-by: Scott Wood <scottwood@freescale.com>
Signed-off-by: David Feng <fenghua@phytium.com.cn>