Updated usage information because of removed short parameters.

Signed-off-by: Robin Schneider <ypid@riseup.net>

contrib/mkimage.sh

@@ -4,7 +4,7 @@ set -e
 mkimg="$(basename "$0")"
 
 usage() {
-	echo >&2 "usage: $mkimg [-d dir] [-t tag] [-c compression] script [script-args]"
+	echo >&2 "usage: $mkimg [-d dir] [-t tag] [--compression algo| --no-compression] script [script-args]"
 	echo >&2 "   ie: $mkimg -t someuser/debian debootstrap --variant=minbase jessie"
 	echo >&2 "       $mkimg -t someuser/ubuntu debootstrap --include=ubuntu-minimal --components=main,universe trusty"
 	echo >&2 "       $mkimg -t someuser/busybox busybox-static"