Platform: corstone1000: BL1 changes to adapt to new flash layout

The commit prepares BL1 to adapt to new GPT-based flash layout.

BL1 does not incorporate a GPT parser and still uses a static
configuration to understand the flash.

The flash_layout.h is also modified/marked in a way to start
the process of its simplification.

Signed-off-by: Satish Kumar <satish.kumar01@arm.com>
Signed-off-by: Mohamed Omar Asaker <mohamed.omarasaker@arm.com>
Change-Id: Ie33803dff9f2729e45ee3b72f694a9929a4395d6