refactor(bl1): clean up bl2 layout calculation

Layout calculation is spread out between core BL1 logic and common
platform code. Relocate these into common platform code so they are
organised logically.

Change-Id: I8b05403e41b800957a0367316cecd373d10bb1a4
Signed-off-by: Harrison Mutai <harrison.mutai@arm.com>
7 files changed