ROMLIB extract functions code from BL images to put them inside ROM.
This has for effect to reduce the size of the BL images.

This patch take this size reduction into consideration to optimize the
memory layout of BL2.
A new "PLAT_ARM_BL2_ROMLIB_OPTIMIZATION" macro is defined and used to
reduce "PLAT_ARM_MAX_BL2_SIZE". This allows to remove the gap between
BL1 and BL2 when ROMLIB is used and provides more room for BL31.

The current memory gain is 0x6000 for fvp and 0x8000 for juno.

Change-Id: I71c2c2c63b57bce5b22a125efaefc486ff3e87be
Signed-off-by: Louis Mayencourt <louis.mayencourt@arm.com>