wpa_supplicant: Add support for pregenerated MAC

wpa_supplicant: Add support for pregenerated MAC

Add new 'mac_addr' policy (3) with which supplicant expects to also
obtain 'mac_value' with pregenerated value of MAC address to be used for
given SSID.

The main difference between this policy and policy 1 is the ability to
control persistence of the MAC address used.  For example if there is
a requirement to always use the same (but random) MAC address for given
SSID (even if user removes/forgets the network) this could be handled
outside of the wpa_supplicant by using some SSID based hashing scheme to
generate MAC (or by just storing the randomly generated one) and
providing it to wpa_supplicant together with mac_addr=3 policy.

Signed-off-by: Andrzej Ostruszka <amo@semihalf.com>