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>
gplugd: u-boot image updated

md5sum u-boot.bin
f1e2b306ba0aa3f5251ba11a31c22a19  u-boot.bin

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