gplugd: Enable SPI flash init & config Env

setenv var_name var_value; saveenv;

with this value of var_name will persist even
after the reset.

Signed-off-by: Yogi P <yogi@vadactro.org.in>
gplugd: Configuring default uboot environment

Configuring default uboot environment for following
things
a) By default boot from usb.
b) To boot from mmc one can just set bootcmd to 'run
   mmc_bootcmd'
c) Configure defaults for ethernet; for tftpboot to
   work.
d) Option to disable ddr2-test.
   Fire 'run ddr-test-disable' to disable ddr test.
e) Enable bootz.
   One can now boot using zImage and uImage.

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