[netdevice] Use link-layer address as part of RNG seed

iPXE currently seeds the random number generator using the system
timer tick count.  When large numbers of machines are booted
simultaneously, multiple machines may end up choosing the same DHCP
transaction ID (XID) value; this can cause problems.

Fix by using the least significant (and hence most variable) bits of
each network device's link-layer address to perturb the random number
generator.  This introduces some per-machine unique data into the
random number generator's seed, and so reduces the chances of DHCP XID
collisions.

This does not affect the ANS X9.82-compatible random bit generator
used by TLS and other cryptography code, which uses an entirely
separate source of entropy.

Originally-implemented-by: Bernhard Kohl <bernhard.kohl@nsn.com>
Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/net/netdevice.c

  */
 int register_netdev ( struct net_device *netdev ) {
 	static unsigned int ifindex = 0;
+	struct ll_protocol *ll_protocol = netdev->ll_protocol;
 	struct net_driver *driver;
+	uint32_t seed;
 	int rc;
 
 	/* Create device name */
 
 	/* Set initial link-layer address, if not already set */
 	if ( ! netdev_has_ll_addr ( netdev ) ) {
-		netdev->ll_protocol->init_addr ( netdev->hw_addr,
-						 netdev->ll_addr );
+		ll_protocol->init_addr ( netdev->hw_addr, netdev->ll_addr );
 	}
 
+	/* Use least significant bits of the link-layer address to
+	 * improve the randomness of the (non-cryptographic) random
+	 * number generator.
+	 */
+	memcpy ( &seed, ( netdev->ll_addr + ll_protocol->ll_addr_len
+			  - sizeof ( seed ) ), sizeof ( seed ) );
+	srand ( rand() ^ seed );
+
 	/* Add to device list */
 	netdev_get ( netdev );
 	list_add_tail ( &netdev->list, &net_devices );