Select cryptographic algorithms at compile time

Allows to disable some cryptographic algorithms by setting make variables
at build time (either from the environment, the command line or by editing
core/lib/libtomcrypt/sub.mk).

For example:
$ make ... CFG_CRYPTO=n
$ make ... CFG_CRYPTO_DES=n

This can reduce the size of the TEE binary as well as its memory footprint:

$ make -j9 PLATFORM=vexpress-qemu_virt DEBUG= all mem_usage
$ grep RAM out/arm32-plat-vexpress/core/tee.mem_usage
RAM Usage        7DF00000 - 7DF39280 size 00039280 229 KiB 58 pages
$ du -h out/arm32-plat-vexpress/core/tee.bin
164K	out/arm32-plat-vexpress/core/tee.bin

$ make -j9 PLATFORM=vexpress-qemu_virt DEBUG= CFG_CRYPTO=n all mem_usage
$ grep RAM out/arm32-plat-vexpress/core/tee.mem_usage
RAM Usage        7DF00000 - 7DF1D280 size 0001D280 117 KiB 30 pages
$ du -h out/arm32-plat-vexpress/core/tee.bin
104K	out/arm32-plat-vexpress/core/tee.bin

Signed-off-by: Jerome Forissier <jerome.forissier@linaro.org>
Tested-by: Jerome Forissier <jerome.forissier@linaro.org> (QEMU virt, FVP)