build(deps): 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.

Actually, the upgrade this time simply boils down to including the new
source code module 'constant_time.c' into the firmware.

To quote mbed TLS v2.28.0 release notes [1]:

  The mbedcrypto library includes a new source code module
  constant_time.c, containing various functions meant to resist timing
  side channel attacks. This module does not have a separate
  configuration option, and functions from this module will be
  included in the build as required.

As a matter of fact, if one is attempting to link TF-A against mbed
TLS v2.28.0 without the present patch, one gets some linker errors
due to missing symbols from this new module.

Apart from this, none of the items listed in mbed TLS release
notes [1] directly affect TF-A. Special note on the following one:

  Fix a bug in mbedtls_gcm_starts() when the bit length of the iv
  exceeds 2^32.

In TF-A, we do use mbedtls_gcm_starts() when the firmware decryption
feature is enabled with AES-GCM as the authenticated decryption
algorithm (DECRYPTION_SUPPORT=aes_gcm). However, the iv_len variable
which gets passed to mbedtls_gcm_starts() is an unsigned int, i.e. a
32-bit value which by definition is always less than 2**32. Therefore,
we are immune to this bug.

With this upgrade, the size of BL1 and BL2 binaries does not appear to
change on a standard sample test build (with trusted boot and measured
boot enabled).

[1] https://github.com/Mbed-TLS/mbedtls/releases/tag/v2.28.0

Change-Id: Icd5dbf527395e9e22c8fd6b77427188bd7237fd6
Signed-off-by: Sandrine Bailleux <sandrine.bailleux@arm.com>

drivers/auth/mbedtls/mbedtls_common.mk

@@ -48,6 +48,7 @@ LIBMBEDTLS_SRCS		:= $(addprefix ${MBEDTLS_DIR}/library/,	\
 					rsa_internal.c				\
 					x509.c 					\
 					x509_crt.c 				\
+					constant_time.c 			\
 					)
 
 # The platform may define the variable 'TF_MBEDTLS_KEY_ALG' to select the key