omap4: allow the use of a plain text env file instead boot scripts

omap4: allow the use of a plain text env file instead boot scripts

For production systems it is better to use script images since
they are protected by checksums and carry valuable information like
name and timestamp. Also, you can't validate the content passed to
env import.

But for development, it is easier to use the env import command and
plain text files instead of script-images.

Since both OMAP4 supported boards (Panda and TI SDP4430) are used
primarily for development, this patch allows U-Boot to load env var
from a text file in case that an boot.scr script-image is not present.

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.

Signed-off-by: Javier Martinez Canillas <javier.martinez@collabora.co.uk>
Acked-by: Nishanth Menon <nm@ti.com>