Always use OMK_*FLAGS instead of *FLAGS

Always use OMK_*FLAGS instead of *FLAGS

We want to stick with the semantics defined by automake. In it *FLAGS are
user variables that should allow the user to override anything. Therefore,
we should not modify them in Makefile.rules at all.

The problem with the previous code was the following:
If config.target defined OMK_CFLAGS=-O0, then Makefile.rules defined
CFLAGS=-Wall -O2 and -O2 overrode the -O0.

This also fixes the test added in the previous commit.