Use SPDX license identifiers ...

To make software license auditing simpler, use SPDX[0] license
identifiers instead of duplicating the license text in every file.

NOTE: Files that have been imported by FreeBSD have not been modified.

[0]: https://spdx.org/

Change-Id: I80a00e1f641b8cc075ca5a95b10607ed9ed8761a
Signed-off-by: dp-arm <dimitris.papastamos@arm.com>

stdlib: add missing features to build PolarSSL ...

This patch adds the missing features to the C library included
in the Trusted Firmware to build PolarSSL:

  - strcasecmp() function
  - exit() function
  - sscanf()* function
  - time.h header file (and its dependencies)

* NOTE: the sscanf() function is not a real implementation. It just
returns the number of expected arguments by counting the number of
'%' characters present in the formar string. This return value is
good enough for PolarSSL because during the certificate parsing
only the return value is checked. The certificate validity period
is ignored.

Change-Id: I43bb3742f26f0bd458272fccc3d72a7f2176ab3d