Implement OpenSSL crypto provider (WITH_CRYPTO=openssl)

Set the WITH_CRYPTO build option to 'openssl' to use the OpenSSL licrypto
library for cryptographic services. When WITH_CRYPTO is not set or set to
'tomcrypt', the default LibTomCrypt library is used.

Usage example:
  $ make CROSS_COMPILE=arm-linux-gnueabihf- \
         PLATFORM=vexpress PLATFORM_FLAVOR=fvp \
         WITH_CRYPTO=openssl

Tested with xtest on Foundation_v8 (PLATFORM_FLAVOR=fvp) and QEMU
(PLATFORM_FLAVOR=qemu_virt).

Signed-off-by: Jerome Forissier <jerome.forissier@linaro.org>
plat-vexpress: eliminate dead code in TEE core at link time

This reduces the size of a nodebug/notrace build by 7%, and by 43% when
WITH_CRYPTO=openssl (as it is, core/lib/openssl/sub.mk brings in a lot of
unused code).