kernel: make Wireless Extensions (WEXT) support optional

kernel: make Wireless Extensions (WEXT) support optional

Currently we always have WEXT support enabled, but it is only needed for
a few legacy drivers and utilities (and, unfortunately the rtl8192cu 11n
staging driver).

Since this bloats both kernel (builtin and in cfg80211) and userspace
(wpa-supplicant) for most targets, allow disabling it for building
leaner kernels.

In a short test build with ALL_NONSHARED on ar71xx, the unpadded file
sizes were:

        y       n       delta
kernel	1364205	1361585 -2620
rootfs	2020282	2006686	-13596

Resulting in a total of 15.8 kB saved.

Signed-off-by: Jonas Gorski <jonas.gorski@gmail.com>