wpa_supplicant: Add an explicit option to enable nl80211.

wpa_supplicant: Add an explicit option to enable nl80211.

Currently, nl80211 support is conditionnal with libnl being enabled,
using implicit dependencies.  This causes problems since it is not
obvious and wpa_supplicant without nl80211 isn't what most user expects.

If nl80211 isn't enabled, then buildroot only enables the wext driver,
which will only work if some deprecated kernel feature isn't left
disabled, or if using a outdated out-of-tree linux driver which doesn't
use the cfg80211 infrastructure.

This makes nl80211 support an explicit option, which
"select BR2_PACKAGE_LIBNL" accordingly.  To handle upgrades nicely, it
would have been nice to have "default y if BR2_PACKAGE_LIBNL", but
Kconfig treats this as a circular dependency.  So instead, this enables
the option by default, which is less worse than not enabling nl80211
when it was previously implicitely enabled.

[Thomas:
 - rewrap Config.in help text
 - add comment about thread dependency.]

Signed-off-by: Nicolas Cavallari <nicolas.cavallari@green-communications.fr>
Reviewed-by: Gary Bisson <gary.bisson@boundarydevices.com>
Tested-by: Gary Bisson <gary.bisson@boundarydevices.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>