aosp_arm64 Copy boot image from $OUT to target files

If BOARD_COPY_BOOT_IMAGE_TO_TARGET_FILES is defined,
in target files, instead of rebuilding the boot image, copy the boot
image already built in $OUT to target files package directly so that
they are the same package.

Define BOARD_COPY_BOOT_IMAGE_TO_TARGET_FILES for aosp_arm64.

The GKI APEX is built using the boot image in $OUT. If the boot image in
$OUT is different from the boot image in target files, aka the generic
boot image we release, the GKI APEX we built is invalid.

If another device needs to copy $OUT/boot.img to target files, it can
define BOARD_COPY_BOOT_IMAGE_TO_TARGET_FILES.

Fixes: 172682114
Test: lunch aosp_arm64 &&
Change-Id: I10fc7a5aa36e976dbeaf25434239687455bba061