mk: influence CPU flags with user input

mk: influence CPU flags with user input

We have encountered a CPU where the AES-NI instruction set is disabled
due to export restrictions. Since the build machine and target machine
is different, using -native configs doesn't work, and on this CPU, the
application refuses to run due to the AES CPU flags being amiss.

The patch passes EXTRA_CFLAGS to the figure-out-cpu-flags helper,
which allows us to add -mno-aes to the compile flags and resolve this
problem.

Signed-off-by: Simon Kagstrom <simon.kagstrom@netinsight.net>
Acked-by: Olivier Matz <olivier.matz@6wind.com>