fix option parsing in the case some CONFIG_x are off.
disable -b on NOMMU, make backgrounding work correctly
(if a bit differently from MMU case). Previously,
it wasn't working at all.
stop using global data for flags in main(), opt
bitfield works as well.
function old new delta
cryptpw_main 177 153 -24
packed_usage 24478 24452 -26
client_background 26 - -26
udhcpc_main 2462 2372 -90
------------------------------------------------------------------------------
(add/remove: 0/1 grow/shrink: 0/3 up/down: 0/-166) Total: -166 bytes