cmd: fdt: skip board specific fixup using env variable

Sometimes it is useful to boot OS with already fixed-up
device tree. Check for env variable 'skip_board_fixup'
before calling ft_board_setup().
Current behaviour is unchanged, additionally user can
set skip_board_fixup to 1 to skip the fixup.

Signed-off-by: Wasim Khan <wasim.khan@nxp.com>