[config] Boards can supply custom super image script

Some boards may need to customize the format of the super image. We will
now provide the variable BOARD_CUSTOM_BUILD_SUPER_IMAGE so that boards
can provide their own script for constructing the super image. This
mimics similar code in the same file for allowing boards to supply a
custom VBMeta (BOARD_CUSTOM_AVBTOOL) or boot image
(BOARD_CUSTOM_MKBOOTIMG).

Bug: None
Change-Id: I461caeb6dfb705fd5c671228fc35561ed403a1cb