utils/genrandconfig: test with BR2_OPTIMIZE_2=y

utils/genrandconfig: test with BR2_OPTIMIZE_2=y

Currently all random defconfigs which are used in autobuilder use size
optimizaion (-Os), since BR2_OPTIMIZE_S=y is the default.

Adding "-O2" optimization will give better test coverage.

In many cases software gets built with speed optimization rather than
size optimization. So let's add Level 2 optimizaion option to be
generated in random defconfigs, so we could be able to test how
packages are built with "-O2" in autobuilder.

Signed-off-by: Evgeniy Didin <Evgeniy.Didin@synopsys.com>
Cc: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
Cc: arc-buildroot@synopsys.com
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>