mesh: Add mesh interface creation command for mesh gate

mesh: Add mesh interface creation command for mesh gate

The mesh gate is used to bridge (or route) between mesh network and
another network. For example, mesh gate acts as router between mesh
network and IEEE 802.11 BSS network.

This command makes a virtual mesh interface to be used for mesh gate.

This command expects to be used like this.

wpa_cli -i wlan0 MESH_INTERFACE_ADD ifname=mesh0
wpa_cli -i mesh0 add_network
wpa_cli -i mesh0 set_network 0 ssid '"commell_2X_mmm"'
wpa_cli -i mesh0 set_network 0 mode 5
wpa_cli -i mesh0 set_network 0 frequency 2412
wpa_cli -i mesh0 set_network 0 key_mgmt SAE
wpa_cli -i mesh0 set_network 0 psk '"01234567"'
wpa_cli -i mesh0 mesh_group_add 0
wpa_cli -i wlan0 mesh_group_remove mesh0

Signed-off-by: Masashi Honma <masashi.honma@gmail.com>