test_firmware: enable custom fallback testing on limited kernel configs

When a kernel is not built with:

CONFIG_HAS_FW_LOADER_USER_HELPER_FALLBACK=y

We don't currently enable testing fw_fallback.sh. For kernels that
still enable the fallback mechanism, its possible to use the async
request firmware API call request_firmware_nowait() using the custom
interface to use the fallback mechanism, so we should be able to test
this but we currently cannot.

We can enable testing without CONFIG_HAS_FW_LOADER_USER_HELPER_FALLBACK=y
by relying on /proc/config.gz (CONFIG_IKCONFIG_PROC), if present. If you
don't have this we'll have no option but to rely on old heuristics for now.

We stuff the new kconfig_has() helper into our shared library as we'll
later expando on its use elsewhere.

Acked-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Luis R. Rodriguez <mcgrof@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>