When Trusted Board Boot is enabled, we need to specify the Chain of
Trust (CoT) of the BL1 and BL2 images. A CoT consists of an array
of image descriptors. The authentication module assumes that each
image descriptor in this array is indexed by its unique image
identifier. For example, the Trusted Boot Firmware Certificate has to
be at index [TRUSTED_BOOT_FW_CERT_ID].

Unique image identifiers may not necessarily be consecutive. Also,
a given BL image might not use all image descriptors. For example, BL1
does not need any of the descriptors related to BL31. As a result, the
CoT array might contain holes, which unnecessarily takes up space in
the BL binary.

Using pointers to auth_img_desc_t structs (rather than structs
themselves) means these unused elements only use 1 pointer worth of
space, rather than one struct worth of space. This patch also changes
the code which accesses this array to reflect the change to pointers.

Image descriptors not needed in BL1 or BL2 respectively are also
ifdef'd out in this patch. For example, verifying the BL31 image is
the responsibility of BL2 so BL1 does not need any of the data
structures describing BL31.

memory diff:
bl1:        bl2:
    text        text
      -20         -20
    bss         bss
      -1463       0
    data        data
      -256        -48
    rodata      rodata
      -5240       -1952
    total       total
      -6979       -2020

Change-Id: I163668b174dc2b9bbb183acec817f2126864aaad
Signed-off-by: Joel Hutton <Joel.Hutton@Arm.com>