busybox: update to 1.31.0

* Update busybox to version 1.31.0.
    New applets: ts, i2ctransfer
    New (restored) feature: error/info levels in syslog messages.
    Leave new features disabled by default.
* Refresh patches
* Remove patch that was backported from upstream

Config refreshed with commands below, after which the OpenWrt specific
config defaults (ipv6, login session child) were corrected:

  make package/busybox/compile   (to populate the build_dir)

  cd package/utils/busybox/config/
  ../convert_menuconfig.pl ../../../../build_dir/target-mips_24kc_musl/busybox-1.31.0

  cd package/utils/busybox
  ./convert_defaults.pl < ../../../build_dir/target-mips_24kc_musl/busybox-1.31.0/.config > Config-defaults.in

Signed-off-by: Hannu Nyman <hannu.nyman@iki.fi>

package/utils/busybox/config/networking/udhcp/Config.in

@@ -104,7 +104,7 @@ config BUSYBOX_CONFIG_UDHCPC_DEFAULT_SCRIPT
 # udhcpc6 config is inserted here:
 config BUSYBOX_CONFIG_UDHCPC6
 	bool "udhcpc6 (21 kb)"
-	default BUSYBOX_DEFAULT_UDHCPC6  # not yet ready
+	default BUSYBOX_DEFAULT_UDHCPC6
 	depends on BUSYBOX_CONFIG_FEATURE_IPV6
 	help
 	udhcpc6 is a DHCPv6 client