board: tbs2910: Enable distro_boot support.

board: tbs2910: Enable distro_boot support.

This keeps the compatibility with the old bootcmd.

The fdtfile environment variable also needed to be set to
imx6q-tbs2910.dtb to enable booting mainline kernels
otherwise with extlinux.conf it tries to load
mx6-tbs2910.dtb instead.

With arm-linux-gnueabi-gcc 9.2.0-1 from the Parabola
GNU/Linux distribution, we have the following size
differences:
- text: +2041 bytes
- data: 0 bytes
- bss: 0 bytes
- total: +2041 bytes

Signed-off-by: Denis 'GNUtoo' Carikli <GNUtoo@cyberdimension.org>
Reviewed-by: Soeren Moch <smoch@web.de>