libpayload: add memchr to libc

libfdt requires memchr. Add missing function to libc.

Change-Id: I872026559d16a352f350147c9d7c4be97456a99f
Signed-off-by: Philipp Hug <philipp@hug.cx>
Reviewed-on: https://review.coreboot.org/c/31354
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Julius Werner <jwerner@chromium.org>

payloads/libpayload/libc/memory.c

@@ -145,3 +145,15 @@ static int default_memcmp(const void *s1, const void *s2, size_t n)
 
 int memcmp(const void *s1, const void *s2, size_t n)
 	__attribute__((weak, alias("default_memcmp")));
+
+
+void *memchr(const void *s, int c, size_t n)
+{
+	unsigned char *p = (unsigned char *)s;
+	while (n--)
+		if (*p != (unsigned char)c)
+			p++;
+		else
+			return p;
+	return 0;
+}