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/coreboot/mkhelper.cfg

@@ -26,3 +26,11 @@ cbfscfg=""
 premake="corebootpremake"
 mkhelper="coreboot_pad_one_byte"
 postmake="mkcorebootbin"
+
+# must confirm, but i think git-clean
+# will also delete crossgcc builds in
+# coreboot.git, which we do not currently
+# rely on out-of-tree
+gitclean="n"
+
+# so, TODO: support out-of-tree xgcc builds