board: tbs2910: Fix video output with existing environments

Migration to DM_VIDEO changed the output device name (in stout, stderr)
from 'vga' to 'vidconsole'. This also was adapted in the default environment.
However, existing user defined environments still use 'vga'. Enable this
workaround to map the 'vga' name to 'vidconsole'. So we get HDMI video
output also with existing legacy environments.

Signed-off-by: Soeren Moch <smoch@web.de>