Inject boot patch level into boot-debug.img

When /boot partition is AVB chained, boot patch level will be
included into the AVB metadata appended to boot.img. We should
also do the same for boot-debug.img to pass keymaster v4 VTS
tests, whichs checks per-partition security patch levels are
indicated in AVB properties.

Bug: 136980825
Test: avbtool info_image --image $OUT/boot-debug.img, checks
      com.android.build.boot.security_patch is there

Change-Id: I2ea1ebfa6abbbbc95639817c208a90642fc3d068