trees: allow global config on multi-tree projects

config/data/PROJECT/mkhelper.cfg can be provided, for
configuration, and it is loaded *before* target.cfg

there are certain instances where we repeat a lot of
config per tree, in multi-tree projects.

for example, we have the exact same config per grub
tree, besides tree name and revision number, for things
like autoconf arguments.

this last problem will be addressed, in a follow-up
patch, and then expanded upon for other projects.

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

script/trees

@@ -143,6 +143,7 @@ load_target_config()
 	eval `setvars "" xarch xlang bootstrapargs autoconfargs xtree \
 	    tree_depend makeargs btype mkhelper`
 	[ -f "$1/target.cfg" ] || btype="auto"
+	[ -f "$datadir/mkhelper.cfg" ] && eval `setcfg "$datadir/mkhelper.cfg"`
 
 	_tcfg="$1/target.cfg"
 	while [ -f "$_tcfg" ] || [ "$cmd" = "build_targets" ]; do