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/lib/libc/libc.mk b/lib/libc/libc.mk
index e1b5560..729b91c 100644
--- a/lib/libc/libc.mk
+++ b/lib/libc/libc.mk
@@ -16,12 +16,14 @@
 			printf.c			\
 			putchar.c			\
 			puts.c				\
+			rand.c				\
 			snprintf.c			\
 			strchr.c			\
 			strcmp.c			\
 			strlcpy.c			\
 			strlen.c			\
 			strncmp.c			\
+			strncpy.c			\
 			strnlen.c			\
 			strrchr.c)