net: Add net_prefix_from_ipv6 utility

Add function that zeroes bits after prefix_len'th bit in an address.  Put
it in net-private.h so it can be used by both netconfig.c and icmp6.c,
potentially others.
diff --git a/ell/net-private.h b/ell/net-private.h
index cc395bb..39d4d98 100644
--- a/ell/net-private.h
+++ b/ell/net-private.h
@@ -22,3 +22,21 @@
 
 char *net_domain_name_parse(const uint8_t *raw, size_t raw_len);
 char **net_domain_list_parse(const uint8_t *raw, size_t raw_len);
+
+static inline const void *net_prefix_from_ipv6(const uint8_t *address,
+						uint8_t prefix_len)
+{
+	uint8_t last_byte = prefix_len / 8;
+	uint8_t bits = prefix_len & 7;
+	static uint8_t prefix[16];
+
+	memcpy(prefix, address, last_byte);
+
+	if (prefix_len & 7) {
+		prefix[last_byte] = address[last_byte] & (0xff00 >> bits);
+		last_byte++;
+	}
+
+	memset(prefix + last_byte, 0, 16 - last_byte);
+	return prefix;
+}