mbedtls: allow to use a platform specific config.h file

A platform specific mebedtls_config.h can now be defined if needed
by the parent makefile.

Signed-off-by: Nicolas Toromanoff <nicolas.toromanoff@st.com>
Change-Id: I532284a72d27424cc801874d7cd0caffe90f2011
Reviewed-on: https://gerrit.st.com/c/mpu/oe/st/tf-a/+/184838


Reviewed-by: CITOOLS <smet-aci-reviews@lists.codex.cro.st.com>
Reviewed-by: Lionel DEBIEVE <lionel.debieve@st.com>
Reviewed-by: Yann GAUTIER <yann.gautier@st.com>
Tested-by: Lionel DEBIEVE <lionel.debieve@st.com>