Create output directory for bin2hex before compiling

If target_firmware/magpie_fw_dev/build/utility/bin is not created
before compiling bin2hex build will fail.

Signed-off-by: Eugene Krasnikov <k.eugene.e@gmail.com>
Merge pull request #9 from KrasnikovEugene/master

Fix for https://github.com/qca/open-ath9k-htc-firmware/issues/8