gplugd: Set MAC address of ethernet based on MFG data.

One time programmable memory can be accessed by
flash reading with command 0x77. This contains
ethernet mac address and serial number.

Use same macaddress for configuring ethaddr
environment variable.

MFG (Manufacturing) data can be seen with custom uboot command
'mfg_data'.

(Based on work of pranav.tipnis@einfochips.com)

Signed-off-by: Yogi P <yogi@vadactro.org.in>