Some platform porting functions were in BL specific header files.
These have been moved to platform.h so that all porting functions
are in the same place. The functions are now grouped by BL.
Obsolete BL headers files have been removed.

Also, the weak declarations of the init_blX_mem_layout() functions
have been moved out the header file and into the source file
(bl_common.c) using the more succinct #pragma syntax. This
mitigates the risk of 2 weak definitions being created and the
wrong one being picked up by the compiler.

Change-Id: Ib19934939fd755f3e5a5a5bceec88da684308a83