common.mk: remove unused targets and definitions

The OP-TEE client, examples, tests (xtest) and benchmark projects are
built using Buildroot. Therefore all the related targets and
definitions in common.mk are unused. Remove them. Similarly, gen_rootfs
can be removed since the root FS is created by Buildroot.

Signed-off-by: Jerome Forissier <jerome@forissier.org>
Reviewed-by: Etienne Carriere <etienne.carriere@linaro.org>