soong: Bring bootloader_message_offset back

This is squashed revert of:

    Revert "config: Remove BOOTLOADER_MESSAGE_OFFSET"

    This reverts commit a8bbd0e4.

    Change-Id: I66df34e901e87e543102377469a150a9bf03e5b8

    Revert "Revert "soong: Export bootloader_message_offset to dependencies""

    This reverts commit d0b4f989.

    Reason for revert: Bringing this back, LGE 8996 lives.

    Change-Id: Iae68fee53ba647d8c64c54388e7296232adf4db8

Change-Id: I358d8e19f2d4da590c9579c1473b5f1964bb71aa