odroid_xu3: Fix board environment variable

When migrating CONFIG_CONS_INDEX to Kconfig, on this platform we changed
what "board" evaluated to in the environment.  This in turn meant that
we would no longer try and find the correct fdtfile via the normal
distro boot logic.  Fix this by overriding board in the default
environment, as done on other platforms where CONFIG_SYS_BOARD is not
what we want to be in the board environment variable.

Fixes: f76750d1 ("Convert CONFIG_CONS_INDEX et al to Kconfig")
Reported-by: Gabriel Hojda <ghojda@yo2urs.ro>
Tested-by: Gabriel Hojda <ghojda@yo2urs.ro>
Signed-off-by: Tom Rini <trini@konsulko.com>