u-boot: Optionally deploy an environment or script file

u-boot: Optionally deploy an environment or script file

Some boards are configured to read a script or environment file as part
of the u-boot boot sequence.  This file, typically called 'uEnv.txt' or
'boot.scr', would be deployed alongside the u-boot binary.  If a recipe
uses this u-boot.inc, such a file can be deployed by setting the optional
UBOOT_ENV parameter and including the file in the SRC_URI.  For example:

    SRC_URI_append_overo = "file://uEnv.txt"
    UBOOT_ENV_overo = "uEnv"

Signed-off-by: Ash Charles <ashcharles@gmail.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>