docs(prerequisites): upgrade to mbed TLS 2.28.0

Upgrade to the latest and greatest 2.x release of Mbed TLS library
(i.e. v2.28.0) to take advantage of their bug fixes.

Note that the Mbed TLS project published version 3.x some time
ago. However, as this is a major release with API breakages, upgrading
to 3.x might require some more involved changes in TF-A, which we are
not ready to do. We shall upgrade to mbed TLS 3.x after the v2.7
release of TF-A.

Change-Id: I887dfd87893169c7be53b986e6c43338d15949d7
Signed-off-by: Sandrine Bailleux <>
1 file changed