task-base: Make "wifi" belong to COMBINED_FEATURES.

task-base: Make "wifi" belong to COMBINED_FEATURES.
* This is consistent with how "bluetooth" is handled already, and, assuming
that distro supports wifi, essentially allows machine to decide if wifi packages
go into rootfs or would be installed from feed. This is helpful for machines
which don't have wifi and devoid of any means of external card extension, or
for machines with very small ROM (like, for example, h3600 with its 16Mb,
in which wifi packages simply don't fit, so h3600 users with CF sleeve and
wifi card would need to install support manually, after deleting packages they
can sacrifice for this).
* Approved by Marcin Juszkiewicz