omap3_beagle: enable the use of a plain text file

Using the new env import command it is possible to use plain text files instead
of script-images. Plain text files are much easier to handle.

E.g. If your boot.scr contains the following:
-----------------------------------
setenv dvimode 1024x768-16@60
run loaduimage
run mmcboot
-----------------------------------
you could create a file named uEnv.txt and use that instead of boot.scr:
-----------------------------------
dvimode=1024x768-16@60
uenvcmd=run loaduimage; run mmcboot
-----------------------------------
The variable uenvcmd (if existent) will be executed (using run) after uEnv.txt
was loaded. If uenvcmd doesn't exist the default boot sequence will be started,
therefore you could just use
-----------------------------------
dvimode=1024x768-16@60
-----------------------------------
as uEnv.txt because loaduimage and mmcboot is part of the default boot sequence

Signed-off-by: Jason Kridner <jkridner@beagleboard.org>
Signed-off-by: Sandeep Paulraj <s-paulraj@ti.com>