libc: Adapt to TFTF

Add support for functions used in TFTF but not in TF-A.

Replaced calls to plat_panic_handler with calls to panic, since there is no
implementation of the former in TFTF.

Change-Id: Ic10de2c6e749db97b932cd7ffbb6067b5befe914
Signed-off-by: Ambroise Vincent <ambroise.vincent@arm.com>
diff --git a/include/lib/libc/stdlib.h b/include/lib/libc/stdlib.h
index edd6265..0ef077f 100644
--- a/include/lib/libc/stdlib.h
+++ b/include/lib/libc/stdlib.h
@@ -19,8 +19,13 @@
 
 #define _ATEXIT_MAX 1
 
+#define	RAND_MAX	0x7ffffffd
+
 extern void abort(void);
 extern int atexit(void (*func)(void));
 extern void exit(int status);
 
+int rand(void);
+void srand(unsigned int seed);
+
 #endif /* STDLIB_H */