tf-a-lts2.8-tftf-gerrit-tforg-l1,2: Set MBEDTLS_URL to 2.28.1

As required for the LTS branch.

Signed-off-by: Paul Sokolovsky <paul.sokolovsky@linaro.org>
Change-Id: If262d22e1af3b94f8df62d2e352b567e86185307
2 files changed