Add BOARD_INIT_BOOT_IMAGE_PAGESIZE

This allows partners to configure the pagesize flag to mkbootimg while
creating the init_boot.img.
The pagesize flag is only used for the padding of the image itself which
is managed in the bootloader.

Flag: EXEMPT bug fix
Test: m # with this defined in cuttlefish
Bug: 411213637
Change-Id: I48da7722c68e3bf3af9ab5af9cff19a11b0820eb