Boot: Mitigate TOCTOU attack when RAM loading

First load the image into RAM, and then perform hash and signature
checks on the RAM image. Replaces verify then copy that was susceptible
to the image being replaced between the operations. Use RAM image to
generate boot record.

Change-Id: I519cf0d5e2757791e9706008caf4098bfe5884c9
Signed-off-by: Raef Coles <raef.coles@arm.com>