Create a library file for libmbedtls

TF Makefile was linking all the objects files generated for the
Mbed TLS library instead of creating a static library that could be
used in the linking stage.

Change-Id: I8e4cd843ef56033c9d3faeee71601d110b7e4c12
Signed-off-by: Roberto Vargas <roberto.vargas@arm.com>