fdt: Fix 'system' command

'fdt systemsetup' wasn't working, due to the fact that the 'set' command
was being parsed in do_fdt() by only testing for the leading 's' instead
of "se", which kept the "sys" test further down from executing. Changed
to test for "se" instead, now 'fdt systemsetup' works (to test the
ft_system_setup proc w/o having to boot a kernel).

Signed-off-by: Tom Warren <twarren@nvidia.com>