libs/light: Rework acpi table build targets

libs/light: Rework acpi table build targets

Currently, a rebuild of libxl will always rebuild "build.o". This is because
the target depends on "acpi" which never exist. So instead we will have
"build.o" have as prerequisites targets that are actually generated by "acpi",
that is $(DSDT_FILES-y).

While "dsdt_*.c" isn't really a dependency for "build.o", a side
effect of building that dsdt_*.c is to also generate the "ssdt_*.h"
that "build.o" needs, but I don't want to list all the headers needed
by "build.o" and duplicate the information available in
"libacpi/Makefile" at this time.

Also avoid duplicating the "acpi" target for Arm, and unique one for
both architecture. And move the "acpi" target to be with other targets
rather than in the middle of the source listing. For the same reason,
move the prerequisites listing for both $(DSDT_FILES-y) and "build.o".

Signed-off-by: Anthony PERARD <anthony.perard@citrix.com>
Acked-by: Andrew Cooper <andrew.cooper3@citrix.com>
Release-acked-by: Henry Wang <Henry.Wang@arm.com>