udhcpc: do not accept --background on NOMMU (same as -b)

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>
diff --git a/networking/udhcp/dhcpc.c b/networking/udhcp/dhcpc.c
index 1ccfff4..6aa6731 100644
--- a/networking/udhcp/dhcpc.c
+++ b/networking/udhcp/dhcpc.c
@@ -73,7 +73,9 @@
 	"request-option\0" Required_argument "O"
 	"no-default-options\0" No_argument   "o"
 	"foreground\0"     No_argument       "f"
+	USE_FOR_MMU(
 	"background\0"     No_argument       "b"
+	)
 	"broadcast\0"      No_argument       "B"
 	IF_FEATURE_UDHCPC_ARPING("arping\0"	Optional_argument "a")
 	IF_FEATURE_UDHCP_PORT("client-port\0"	Required_argument "P")