Although at first glance it seems that with the code it is easy to override Mk/Uses/*.mk with something from ${OVERLAYS}/k/Uses/*.mk practically it is not possible with the current code. For example if we want to override something from the Mk/Uses/php.mk using ${OVERLAYS}/MkUses/php.mk it is not possible with the current code as both files are included with the for loop and it actually overrides with the original Mk/Uses/php.mk as USESDIR takes place in a later order.
This patch at first tries to find whether if there is a file named ${OVERLAYS}/Mk/Uses/*.mk. If not it falls back to ${USESDIR}/Mk/Uses/*.mk. And if not found as usual it throws the error Unknown USES=.