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

Create local C library implementation (2/2) ...

- This change is split into two separate patches in order to
  simplify the history as interpreted by 'git'. The split is
  between the move/rename and addition of new files.
- Remove dependency on toolchain C library headers and functions in
  order to ensure behavioural compatibility between toolchains.
- Use FreeBSD as reference for C library implementation.
- Do not let GCC use default library include paths.
- Remove unused definitions in modified headers and implementations.
- Move C library files to 'lib/stdlib' and 'include/stdlib'.
- Break std.c functions out into separate files.

Change-Id: I3e3d8d992052264d2a02489034ae4c03bf0f5512