Use constant-time comparison for MAC verification #17

Job	Run time
build (Heltec_v3_room_server)	2m 50s
build (Heltec_v3_companion_radio_ble)	1m 49s
build (PicoW_repeater)	2m 40s
build (RAK_4631_companion_radio_ble)	1m 37s
build (Heltec_v3_repeater)	1m 52s
build (RAK_4631_room_server)	1m 35s
build (LilyGo_Tlora_C6_repeater_)	1m 50s
build (RAK_4631_repeater)	1m 25s
build (wio-e5-mini_repeater)	1m 26s
	17m 4s

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