Boot: Add constant time memory equality function

Add boot_secure_memequal function which runs in constant time,
mitigating the risk of timing side channel attacks. Replace calls to
memcmp where applicable (where they test only equality).

Change-Id: I062a433a67a1a865d6e890ba06a75bfb6b13deb3
Signed-off-by: Raef Coles <raef.coles@arm.com>