build: Add support for assembling multi-image uboot files

Change-Id: I7300d36cc4227ada77f1800680869037ef33c5e8
build:  Add USE_SET_METADATA tag

On some of the older devices the recovery doesn't support
metadata when setting permissions.  This allows to use
set_perm vs set_metadata you will need to set
USE_SET_METADATA := false inside your boardconfig.mk

Change-Id: Ib6cf97b5ca28cee8a87975231f3a61b156e77d29