busybox: add new 1.20.0, remove deprecated, mark 1.18.x deprecated

Add version 1.20.0, remove deprecated 1.16.x & 1.17.x versions and mark
1.18.x as deprecated.

Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>