Show mk2rbc warnings when they occur

There was a missing $ for variable expansion,
and $(info) does not seem to show up in soong output,
but $(warning) does.

Fixes: 201431870
Test: After introducing a problem in mk2rbc: m RBC_PRODUCT_CONFIG=1 nothing
Change-Id: Ife7786d27a7315e873ea125ca361e684ffb79e1e