Rework image code to handle no results from YAML

When grep has 0 matches it returns a non-zero exit code. In this case
a 0 match grep is a possibility for YAML files so ignore grep's exit
code in this instance.

Change-Id: Ia7eb55364feeb42bf35681c4ae0798fe25bbfad1
Signed-off-by: Thanh Ha <thanh.ha@linuxfoundation.org>