Add and use new ModeReason::AUX_FUNCTION #39203

Job	Run time
build (stm32f7, chibios, 10)	8m 10s
build (stm32h7, chibios, 10)	7m 6s
build (fmuv2-plane, chibios, 10)	3m 18s
build (periph-build, chibios, 10)	3m 38s
build (iofirmware, chibios, 10)	1m 32s
build (CubeOrange-bootloader, chibios, 10)	57s
build (fmuv3-bootloader, chibios, 10)	59s
build (revo-bootloader, chibios, 10)	51s
build (stm32h7-debug, chibios, 10)	4m 11s
build (fmuv3, chibios, 10)	10m 5s
build (revo-mini, chibios, 10)	7m 39s
build (MatekF405-Wing, chibios, 10)	8m 16s
build (CubeOrange-ODID, chibios, 10)	5m 12s
build (CubeRedPrimary-bootloader, chibios, 10)	51s
build (configure-all, chibios, 10)	11m 12s
build (build-options-defaults-test, chibios, 10)	14m 17s
build (signing, chibios, 10)	4m 32s
build (CubeOrange-PPP, chibios, 10)	3m 52s
build (SOHW, chibios, 10)	7m 16s
build (Pixhawk6X-PPPGW, chibios, 10)	1m 25s
build (new-check, chibios, 10)	9m 25s
	1h 54m 44s

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