config: qemu: Add QEMU_EXTRA_FLAGS config option

Add QEMU_EXTRA_FLAGS as QEMU board config option.

This allows Twister tests to provide additional device setup
commands to QEMU in prj.conf or testcase.yaml configuration files.

Example use case: to setup TCP or UDP network interfaces
with non-conflicting port numbers in different test suites
to avoid conflicts when Twister run tests in parallel on the
same host.

Signed-off-by: Dmitrii Golovanov <dmitrii.golovanov@intel.com>