xbmk: don't call mkdir. use xbmkdir (new function)

xbmkdir checks if a directory exists, before running
mkdir, and then still uses -p

i was testing xbmk on arch linux today, and noticed
that it errored out when a directory already exists.

i'm mitigating against buggy or differently behaving
mkdir implementations this way, by wrapping around
it.

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

include/init.sh

@@ -58,7 +58,7 @@ xbmk_init()
 	fi
 
 	export PWD="$xbmkpwd"
-	x_ mkdir -p "$basetmp"
+	x_ xbmkdir "$basetmp"
 
 	for init_cmd in get_version set_env set_threads git_init child_exec; do
 		if ! xbmk_$init_cmd "$@"; then