util/genbuild_h: Update IASL location finding code

util/genbuild_h: Update IASL location finding code

Update the iasl path finding code to use XGCCPATH if it's set, and to
look for iasl on the path if it's not set and not under util/crossgcc.

On the jenkins builders, iasl is in the path, not in util/crossgcc/xgcc.

On the systems of people who have multiple copies of coreboot, it makes
sense to just have a single copy of the toolchain and define XGCCPATH in
the environment to point to it.

Previously, either of these situations resulted in a warning from the
genbuild_h tool that iasl was not found under util/crossgcc, which was
true, but not particularly relevant, and generated confusion.

If xcompile already existed before make was run, the correct path would
be found, but on an initial build, this check couldn't find iasl.

BUG=None
TEST=Build with iasl in /util/crossgcc/xgcc/bin, in the path and in a
directory pointed to with XGCCPATH.

Signed-off-by: Martin Roth <martin@coreboot.org>
Change-Id: Ic2f8dca0be8bfb54d3c672fab6cf6f005bb394c3
Reviewed-on: https://review.coreboot.org/c/coreboot/+/54001
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Angel Pons <th3fanbus@gmail.com>
Reviewed-by: Patrick Georgi <pgeorgi@google.com>