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>