libc: Fix some MISRA defects ...

No functional changes.

Change-Id: I907aa47565af2a6c435a5560041fd2b59e65c25c
Signed-off-by: Antonio Nino Diaz <antonio.ninodiaz@arm.com>

Add atexit function to libc ...

We had exit but we didn't have atexit, and we were calling panic and
tf_printf from exit, which generated a dependency from exit to them.
Having atexit allows to set a different function pointer in every image.

Change-Id: I95b9556d680d96249ed3b14da159b6f417da7661
Signed-off-by: Roberto Vargas <roberto.vargas@arm.com>

Create a library file for libc ...

TF Makefile was linking all the objects files generated for the
c library instead of creating a static library that could be
used in the linking stage.

Change-Id: I721daea097e9b13cbb42c9f8eaa2af8fea0799cf
Signed-off-by: Roberto Vargas <roberto.vargas@arm.com>