The pre-processor directives make it hard to read the non-TBB version of
this function. Refactor the code to improve readability. No functional
change introduced.

In particular, introduce a new helper function load_image_flush(),
that simply loads an image and flushes it out to main memory. This is
the only thing load_auth_image_internal() needs to do when TBB is
disabled or when authentication is dynamically disabled.

In other cases, we need to recursively authenticate the parent images up
to the root of trust. To make this clearer, this code gets moved to a
TBB-specific helper function called load_auth_image_recursive().

As a result, load_auth_image_internal() now boils down to calling the
right helper function (depending on TBB enablement and dynamic
authentication status).

Change-Id: I20a39a3b833810b97ecf4219358e7d2cac263890
Signed-off-by: Sandrine Bailleux <sandrine.bailleux@arm.com>