Remove the wrong extra assertion for size and check size for hashing

This change fixes the following warning message:
  WARNING: out/target/product/$(TARGET_DEVICE)/vendor_boot$(DEBUG TYPE).img approaching
  size limit (67108864 now; limit 67108864)

Because AVB signing will increase boot.img to
BOARD_BOOTIMAGE_PARTITION_SIZE, the assertion should be done before AVB
signing.

And this change also changes the max allowed size to be (partition
size - AVB_HASH_META_SIZE) when AVB is enabled.

This is ported from commit [1] for vendor_boot.img.

[1] commit 03b9c8e6
("Reorder assert-max-image-size and AVB signing")

Bug: 431284150
Change-Id: Ib6e5212d11537ef365907cbe23d30a413c2fd59a
Signed-off-by: Yuan-Jen (淵仁) Cheng <cyuanjen@google.com>