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/mrc.sh

@@ -22,7 +22,7 @@ extract_refcode()
 		err "cbfstoolref not set" "extract_refcode" "$@"
 	fi
 
-	x_ mkdir -p "${_pre_dest%/*}"
+	x_ xbmkdir "${_pre_dest%/*}"
 
 	x_ "$cbfstoolref" "$appdir/bios.bin" extract \
 	    -m x86 -n fallback/refcode -f "$appdir/ref" -r RO_SECTION