Various improvements/cleanups on the linker scripts

  - Check at link-time that bootloader images will fit in memory
    at run time and that they won't overlap each other.
  - Remove text and rodata orphan sections.
  - Define new linker symbols to remove the need for platform setup
    code to know the order of sections.
  - Reduce the size of the raw binary images by cutting some sections
    out of the disk image and allocating them at load time, whenever
    possible.
  - Rework alignment constraints on sections.
  - Remove unused linker symbols.
  - Homogenize linker symbols names across all BLs.
  - Add some comments in the linker scripts.

Change-Id: I47a328af0ccc7c8ab47fcc0dc6e7dd26160610b9