net: Separate ArpRequest() into lower-level func

Link-local support will need to send ARP packets, but needs more
fine-grained control over the contents.  Split the implementation
into 2 parts so link-local can share the code.

Signed-off-by: Joe Hershberger <joe.hershberger@ni.com>
diff --git a/net/arp.h b/net/arp.h
index 956fc5e..bfd57e0 100644
--- a/net/arp.h
+++ b/net/arp.h
@@ -22,6 +22,8 @@
 
 void ArpInit(void);
 void ArpRequest(void);
+void arp_raw_request(IPaddr_t sourceIP, const uchar *targetEther,
+	IPaddr_t targetIP);
 void ArpTimeoutCheck(void);
 void ArpReceive(struct ethernet_hdr *et, struct ip_udp_hdr *ip, int len);