nl80211: Add vendor command support

nl80211: Add vendor command support

Add a callback to the driver interface that allows vendor specific
commands to be sent. In addition, a control interface command is added
to expose this new interface outside wpa_supplicant:

Vendor command's format:
VENDOR <vendor id> <sub command id> [<hex formatted data>]

The 3rd argument will be converted to binary data and then passed as
argument to the sub command.

This interface is driver independent, but for now, this is only
implemented for the nl80211 driver interface using the cfg80211 vendor
commands.

Signed-off-by: Beni Lev <beni.lev@intel.com>