system-arm: Rework boot loader makefile to be more generic

system-arm: Rework boot loader makefile to be more generic

add all, clean and install rules
use variables for CROSS_COMPILE, CC, LD, DESTDIR
use generic rules to produce objects and link

Signed-off-by: Bertrand Marquis <bertrand.marquis@arm.com>
[ciro.santilli@arm.com: Also add BUILDDIR and to allow fully
 out-of-tree builds.]
Signed-off-by: Ciro Santilli <ciro.santilli@arm.com>
Change-Id: Id84bc6a8e5dde409b6fb968925ca268376730196
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/22823
Reviewed-by: Giacomo Travaglini <giacomo.travaglini@arm.com>
Maintainer: Giacomo Travaglini <giacomo.travaglini@arm.com>
Tested-by: kokoro <noreply+kokoro@google.com>