Add BOARD_EXCLUDE_KERNEL_FROM_RECOVERY_IMAGE.

BOARD_EXCLUDE_KERNEL_FROM_RECOVERY_IMAGE removes kernel from the
recovery image.

Test: build recovery image and unpack_bootimg
Test: build target files and unpack_bootimg IMAGES/recovery.img
      on device with GKI and dedicated recovery partition.

Bug: 156098440

Change-Id: I5f37d74ed954b26fe3dd778708b6ab9cfdc51f1e