Bluetooth: Mesh: Add macro control whether adv main to relay

Enable advertiser main to support relay messages, this maximizes
theutilization efficiency of advertising sets, which is helpful for the
sending of dense relays, but for Friend and LPN, it delays the sending
of local messages a little, this may cause the time slot deviation between
friend and lpn, and the power consumption of lpn will increase.

Signed-off-by: Lingao Meng <menglingao@xiaomi.com>