An image descriptor contains an action and an argument.  The action
indicates the intended operation, as requested by the user.  It can be
pack, unpack or remove.  Factor out setting those fields to a separate
function to minimize code duplication across the various commands that
modify these fields.

Change-Id: I1682958e8e83c4884e435cff6d0833c67726461f
Signed-off-by: dp-arm <dimitris.papastamos@arm.com>