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>
diff --git a/bl2u/bl2u_main.c b/bl2u/bl2u_main.c
index a7e3fb9..b29d57e 100644
--- a/bl2u/bl2u_main.c
+++ b/bl2u/bl2u_main.c
@@ -17,6 +17,7 @@
 #include <platform_def.h>
 #include <stdint.h>
 
+
 /*******************************************************************************
  * This function is responsible to:
  * Load SCP_BL2U if platform has defined SCP_BL2U_BASE