busybox: adjust download mirror

* Adjust download locations:
  - use https as busybox.net permanently redirects http to https
  - gentoo mirror has neither 1.25.0 nor 1.25.1 available, so drop it
    in favor of buildroot.net that has 1.25.1

Signed-off-by: Hannu Nyman <hannu.nyman@iki.fi>
Signed-off-by: Stijn Tintel <stijn@linux-ipv6.be>