trees: set btype if target.cfg doesn't exist

set it to "auto", because otherwise it'll be unset,
which means that kconfig type is assumed.

the build system is designed in such a way that multi-tree
is assumed, if the target build system uses kconfig files.

target.cfg is optional on single-tree but not multi-tree,
so it's ok to set something here.

basically, kconfig-type projects will never be single-tree.

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

script/trees

@@ -154,6 +154,7 @@ load_project_config()
 {
 	eval `setvars "" xarch xlang tree bootstrapargs autoconfargs xtree \
 	    tree_depend makeargs btype`
+	[ -f "$1/target.cfg" ] || btype="auto"
 	eval `setcfg "$1/target.cfg" 0`
 	[ -z "$btype" ] || [ "${mode%config}" = "$mode" ] || \
 	    return 1; return 0