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/release.sh

@@ -52,7 +52,7 @@ release()
 	fi
 	x_ rm -Rf "$rsrc"
 
-	x_ mkdir -p "$reldir"
+	x_ xbmkdir "$reldir"
 	x_ mv "$vdir" "$reldir"
 	x_ rm -Rf "${vdir%"/$version"}"