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/vendor.sh

@@ -217,7 +217,7 @@ fetch()
 		return 0
 	fi
 
-	x_ mkdir -p "${_dest%/*}"
+	x_ xbmkdir "${_dest%/*}"
 
 	if [ "$dl_type" != "fsp" ]; then
 		extract_archive "$_dl" "$appdir" || \