busybox: add 1.19.x branch and cleanup

Add busybox version 1.19.2 for the 1.19.x branch.
Remove deprecated 1.15.x branch.
Mark 1.16.x and 1.17.x branches deprecated as suggested by Thomas.

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