build-tf-a.sh: add mbed_tls directory path for clean operation.

If delegated attestation support is enabled for a platform, it would
include mbedtls makefile which expects the MBEDTLS_DIR path to set
explicitly even for the clean operation.

Signed-off-by: Sayanta Pattanayak <sayanta.pattanayak@arm.com>
Change-Id: I1c6cf96d6de1ce7f3c9d07f8ed27605544559b20