fwts: Enable optional efi_runtime kernel module build

Firmware test suite does provides efi_runtime kernel module required
to run UEFI tests. So optionally enable this module build.

[Peter: fix -/_ in comment as suggested by Erico]
Signed-off-by: Sumit Garg <sumit.garg@linaro.org>
Reviewed-by: Erico Nunes <nunes.erico@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>