TF-A: Download mbedTLS if needed, set build dir

Fix TF-A recipe to only download mbedTLS when it is activated.
Allow user to modify TFA_BUILD_DIR and set BUILD_PLAT accordingly in
make arguments to properly set the build directory.

Change-Id: Ib53b245d4d3b2d62e41f0ef238ad03dc45bab056
Signed-off-by: Bertrand Marquis <bertrand.marquis@arm.com>