Support vendor element configuration for AP mode from wpa_supplicant

Support vendor element configuration for AP mode from wpa_supplicant

Support adding/deleting vendor elements dynamically for AP mode while it
is started by wpa_supplicant instead of hostapd which already supported
this. This adds ap_assocresp_elements global parameter and UPDATE_BEACON
control interface command to take the changed values into effect.

Usage in wpa_cli:
Add vendor IE for (Re)Association Response frames
> set ap_assocresp_elements=xxxx
Add vendor IE for Beacon/Probe Response frames
> set ap_vendor_elements=xxxx

Delete vendor IE from (Re)Association Response frames
> set ap_assocresp_elements
Delete vendor IE from Beacon/Probe Response frames
> set ap_vendor_elements

To make vendor IE changes take effect
> update_beacon

Signed-off-by: Chaoli Zhou <zchaoli@codeaurora.org>