ARM: boot: add strrchr function

libfdt gained a new dependency on strrchr, so copy the implementation
from lib/string.c.

Cc: Russell King <linux@armlinux.org.uk>
Cc: linux-arm-kernel@lists.infradead.org
Signed-off-by: NRob Herring <robh@kernel.org>

arch/arm/boot/compressed/string.c

@@ -121,6 +121,16 @@ char *strchr(const char *s, int c)
 	return (char *)s;
 }
 
+char *strrchr(const char *s, int c)
+{
+	const char *last = NULL;
+	do {
+		if (*s == (char)c)
+			last = s;
+	} while (*s++);
+	return (char *)last;
+}
+
 #undef memset
 
 void *memset(void *s, int c, size_t count)