Makefile: Add $(xcompile) to specify where to write xcompile

Makefile: Add $(xcompile) to specify where to write xcompile

This file was being written to the root src directory. It is the only
file being written to src during a normal build, while all others are
being written to $(obj). I added a new variable to allow specifying the
xcompile path. This allows generating a single file if building multiple
boards. I also moved the default location into $(obj) so we don't
pollute the src directory by default.

I also cleaned up the generation of xcompile by removing the unnecessary
eval and NOCOMPILE check.

I also left .xcompile in distclean so it cleans up stale files.

Since .xcompile is written into $(obj), `make clean` will now remove it.

The tegra Makefiles are outside of the normal build process, so I just
updated those Makefiles to point to the default xcompile location of a
normal build. The what-jenkins-does target had to be updated to support
these special targets. We generate an xcompile specifically for these
targets and pass it into the Makefile. Ideally we should get these
targets added to the main build.

BUG=b:112267918
TEST=ran `emerge-grunt coreboot` and `make what-jenkins-does`

Signed-off-by: Raul E Rangel <rrangel@chromium.org>
Change-Id: Ia83f234447b977efa824751c9674154b77d606b0
Reviewed-on: https://review.coreboot.org/c/coreboot/+/28101
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Nico Huber <nico.h@gmx.de>