system-arm: Rename ARM bootloader source

system-arm: Rename ARM bootloader source

The AArch32 assembly source has been renamed from simple.S to boot.S,
and the Makefile has been renamed to makefile (lowercase) to match
the AArch64 convention

Change-Id: Ia4581fe0223c156460edcc558622b5d7962258dc
Signed-off-by: Giacomo Travaglini <giacomo.travaglini@arm.com>
Reviewed-by: Adrian Herrera <adrian.herrera@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/23949
Reviewed-by: Nikos Nikoleris <nikos.nikoleris@arm.com>
Tested-by: kokoro <noreply+kokoro@google.com>