build-tf-a.sh: add support for RESET_TO_BL31

In normal boot flow, MBEDTLS_DIR is passed to TF-A with TBBR related
flags but TBBR is disabled for RESET_TO_BL31. Hence pass MBEDTLS_DIR
separately with build flags.

Signed-off-by: Rakshit Goyal <rakshit.goyal@arm.com>
Change-Id: I0213f738ac283e0809408d21f00f5e3fb193e80a