udhcpc: kill undocumented -W, it was a no-op.
 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

diff --git a/applets/usage.c b/applets/usage.c
index 4f6a569..a35817f 100644
--- a/applets/usage.c
+++ b/applets/usage.c
@@ -5,6 +5,17 @@
  * object directory */
 #include "../include/autoconf.h"
 
+/* Since we can't use platform.h, have to do this again by hand: */
+#if ENABLE_NOMMU
+#define BB_MMU 0
+#define USE_FOR_NOMMU(...) __VA_ARGS__
+#define USE_FOR_MMU(...)
+#else
+#define BB_MMU 1
+#define USE_FOR_NOMMU(...)
+#define USE_FOR_MMU(...) __VA_ARGS__
+#endif
+
 static const char usage_messages[] = ""
 #define MAKE_USAGE
 #include "usage.h"