environment: distro: Add SF distro command

Add distro boot command support for SPI flash (SF).

This distro boot will read the boot script at specific
location at the flash and start sourcing the same.

This file need to include on required include/config
file.

Cc: Tom Rini <trini@konsulko.com>
Signed-off-by: Jagan Teki <jagan@amarulasolutions.com>
Reviewed-by: Tom Rini <trini@konsulko.com>