libpayload: Move .xcompile into $(obj)

libpayload: Move .xcompile into $(obj)

Keep libpayload's xcompile in its build dir. While we are at it,
align things with the top-level version.

Having `.xcompile` in a central place led to race conditions when
multiple payloads try to build their own libpayloads in parallel.

Change-Id: I504e1862db79b368289867f7568c9169f27a1549
Signed-off-by: Nico Huber <nico.h@gmx.de>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/47651
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Raul Rangel <rrangel@chromium.org>
Reviewed-by: Angel Pons <th3fanbus@gmail.com>