Source img-functions into common-functions.

A recent commit added a call to function cleanup_dirs to
the common-functions file, but this function is not in scope
for all code paths.

The problem can be reproduced like so:
  bin/ramdisk-image-create deploy -o /tmp/foo

Which results in the error:
  bin/../lib/common-functions: line 35: cleanup_dirs: command not found

This change causes cleanup_dirs to be in scope when calling ramdisk-image-create.

Change-Id: Iaf01beb9dd42a9d810d8aed1aa4ce19a5854c09b