Enable mk2rbc warnings/errors by default

Since we are going to be asking partners to start
ensuring that their makefiles work with mk2rbc,
enable warnings by default so they're more visible.
Otherwise, the conversion could appear to succeed
but have some subtle failures.

Bug: 204062171
Test: Manually
Change-Id: I38189031d7357bb3f2c811de90f2821cde10cbcd