lrdpv2: ramdump: return a string from `hexdump'

hexdump currently prints to stdout.  Although that's convenient for
interactive debugging it's not very useful for re-use in other parts of
the parser.  Return a string instead of printing to stdout.

Also add a usage example.

Change-Id: I18535f5339be85ffc5661e51430bba9a55fe665d