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

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

Adapted from d70f5480 described below.

    commit d70f54808dfa83b574e1239c3eccbcf3317343e1
    Author: Javier Martinez Canillas <javier@dowhile0.org>
    Date:   Mon Jan 7 03:51:20 2013 +0000

    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>
Signed-off-by: Peter A. Bigot <pab@pabigot.com>
Tested-by: Ash Charles <ash@gumstix.com>