busybox: background udhcpc by default if no lease is obtained

The current behavior for busybox' udhcpc is to terminate if no lease is
obtained at start up. Therefore no address is acquired if the link is
established afterwards.

By setting the -b flag udhcpc will background allowing the link to be
established at any time.

Signed-off-by: Lothar Felten <lothar.felten@gmail.com>
[Peter: drop incompatible -n, also fixup busybox-minimal.config]
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

package/busybox/busybox-minimal.config

@@ -961,7 +961,7 @@ CONFIG_UDHCP_DEBUG=9
 CONFIG_UDHCPC_SLACK_FOR_BUGGY_SERVERS=80
 # CONFIG_FEATURE_UDHCP_RFC3397 is not set
 # CONFIG_FEATURE_UDHCP_8021Q is not set
-CONFIG_IFUPDOWN_UDHCPC_CMD_OPTIONS="-R -n"
+CONFIG_IFUPDOWN_UDHCPC_CMD_OPTIONS="-b -R"
 
 #
 # Print Utilities

package/busybox/busybox.config

@@ -970,7 +970,7 @@ CONFIG_UDHCP_DEBUG=0
 CONFIG_UDHCPC_SLACK_FOR_BUGGY_SERVERS=80
 CONFIG_FEATURE_UDHCP_RFC3397=y
 CONFIG_FEATURE_UDHCP_8021Q=y
-CONFIG_IFUPDOWN_UDHCPC_CMD_OPTIONS="-R -n -O search"
+CONFIG_IFUPDOWN_UDHCPC_CMD_OPTIONS="-b -R -O search"
 
 #
 # Print Utilities