[ethernet] Provide eth_random_addr() to generate random Ethernet addresses

Modified-by: Michael Brown <mcb30@ipxe.org>
Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/include/ipxe/ethernet.h

 		      const void **ll_dest, const void **ll_source,
 		      uint16_t *net_proto, unsigned int *flags );
 extern void eth_init_addr ( const void *hw_addr, void *ll_addr );
+extern void eth_random_addr ( void *hw_addr );
 extern const char * eth_ntoa ( const void *ll_addr );
 extern int eth_mc_hash ( unsigned int af, const void *net_addr,
 			 void *ll_addr );

src/net/ethernet.c

 FILE_LICENCE ( GPL2_OR_LATER );
 
 #include <stdint.h>
+#include <stdlib.h>
 #include <stdio.h>
 #include <string.h>
 #include <byteswap.h>
 	memcpy ( ll_addr, hw_addr, ETH_ALEN );
 }
 
+/**
+ * Generate random Ethernet address
+ *
+ * @v hw_addr		Generated hardware address
+ */
+void eth_random_addr ( void *hw_addr ) {
+	uint8_t *addr = hw_addr;
+	unsigned int i;
+
+	for ( i = 0 ; i < ETH_ALEN ; i++ )
+		addr[i] = random();
+	addr[0] &= ~0x01; /* Clear multicast bit */
+	addr[0] |= 0x02; /* Set locally-assigned bit */
+}
+
 /**
  * Transcribe Ethernet address
  *