Add FWU booting instructions to the user guide

FWU uses additional images that have to be loaded,
and this patch adds the documentation of how to do
it in FVP and Juno.

Change-Id: I1a40641c11c5a4c8db0aadeaeb2bec30c9279e28
Signed-off-by: Roberto Vargas <roberto.vargas@arm.com>
1 file changed