add support to build tftf test image

TFTF is test suite provided by TF-A team to test interface the TF-A provides to non-secure software component. In case of corstone1000, uboot is replaced with tftf.bin, meaning using has to run following commands to build corstone1000 stack with TFTF test enabled $ kas shell meta-arm/kas/corstone1000-fvp.yml:meta-arm/kas/tftf.yml $ bitbake corstone1000-fipimage-tftf