mk: turn on git-clean by default, except coreboot

libreboot's policy is to thoroughly clean sources per
build, because we cache builds out of tree and avoid
re-builds if they exist; these and the sources are
deleted if metadata in config/ changes per project,
tree and/or target.

the problem is that this change only affects lbmk.git

it doesn't affect release archives, where project sources
do not have git-init applied to them. this will be
addressed.

can't use it on coreboot, because it will delete xgcc
tarballs in a way currently not handled, and handling
it will mean adding a project-specific hack.

the xgcc tarballs could be built out-of-tree, and this
will have to be done, otherwise coreboot will have to be
made gitclean="y" for now.

everything else is fair game.

Signed-off-by: Leah Rowe <leah@libreboot.org>

config/data/u-boot/mkhelper.cfg

@@ -1,4 +0,0 @@
-# SPDX-License-Identifier: GPL-3.0-or-later
-
-gitclean="y"
-gitcleanargs="-fdx"