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Add extern declarations of linker symbols to bl_common.h. These are
used by platform ports to determine the memory layout of BL images.
Adding the declarations to this file facilitates removal of these
declarations from the platform porting source files in subsequent
patches.

Also remove the linker symbol declarations from common TSP source
code.

Change-Id: I8ed0426bc815317c4536b588e4e78bc15b4fe91c