Ensure that IG (Global) bit is reset in MAC address for K64F and others

I can't get a DHCP address because the MAC address chosen by the mbed library has the IG bit set. My DHCP server doesn't respond with an IP address.

Changing the resultant MAC address from the 0x03 to 0x02 allows me to get an IP address.

See IEE 802.3-2002, Section 3.2.3(b)

Here's a Wireshark analysis of the DHCP discover packet with the unmodified code:

![image](https://cloud.githubusercontent.com/assets/99482/4940832/36cba0ba-65de-11e4-8b19-23ea61be4ca0.png)