ramdisk-image-create shows a failed binary dependency

This patch fixes a problem: if a binary is not found, the script
exits without a message informs what is needed (because of 'set -e').

Change-Id: I00b8917918f0e6eddf8506d6548432077eb4bf14

lib/ramdisk-functions

@@ -40,8 +40,7 @@ function ensure_binaries() {
   done
 
   for _BIN in $BINARY_DEPS ; do
-    _LOCATION=""
-    _LOCATION="${_LOCATION:-$(which $_BIN)}"
+    _LOCATION=$(which "$_BIN" || echo "")
     if [ -z "$_LOCATION" ]; then
       echo "$_BIN is not found in your PATH" 1>&2
       echo "Please install it on your system"