libutils: add functions needed by OpenSSL libcrypto

This commit adds the following standard functions to libutils
- abort(), time() [for TEE core only]
- isdigit(), isspace(), isalpha(), isalnum(), isxdigit(), isupper(),
  islower(), toupper(), tolower(): basic ASCII-only implementation
- atoi(), sscanf(), strtoul(), strncmp(), strcpy(), strncpy(), strchr(),
  strcat()
Add support for $(lib-use-ld) and $(lib-ldflags)

When building a static library, set lib-use-ld := y to use $(LD) instead of
$(AR). Link flags may be passed in $(lib-ldflags).

Signed-off-by: Jerome Forissier <jerome.forissier@linaro.org>
Implement OpenSSL crypto provider (WITH_CRYPTO=openssl)

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