Build: Move Storage asset defs to build variables

Removes the MAX_ASSET_SIZE and NUM_ASSETS storage definitions from
flash_layout.h and adds replacement build variables for them. This
allows for greater customisation at build time and reduces duplication
between platforms.

Change-Id: If5d20ee0cb8dede28513c10572da594c02b4ee89
Signed-off-by: Jamie Fox <jamie.fox@arm.com>
27 files changed