Introduce include/environment/ti/dfu.h that
consolidates environment variable definitions
for various TI boards that support DFU today.

Tested on AM335x EVM, AM437x SK EVM and DRA74x
EVM by using DFU to write to SD card.
Signed-off-by: NSekhar Nori <nsekhar@ti.com>
Reviewed-by: NTom Rini <trini@konsulko.com>