Copter: Add 'Unknown' to the flight mode #42807

Job	Run time
build (stm32f7, chibios, 10)	6m 22s
build (stm32h7, chibios, 10)	6m 13s
build (fmuv2-plane, chibios, 10)	3m 56s
build (periph-build, chibios, 10)	5m 22s
build (iofirmware, chibios, 10)	3m 26s
build (CubeOrange-bootloader, chibios, 10)	2m 56s
build (fmuv3-bootloader, chibios, 10)	3m 10s
build (revo-bootloader, chibios, 10)	2m 53s
build (stm32h7-debug, chibios, 10)	4m 43s
build (fmuv3, chibios, 10)	8m 56s
build (revo-mini, chibios, 10)	7m 4s
build (MatekF405-Wing, chibios, 10)	7m 39s
build (CubeOrange-ODID, chibios, 10)	5m 5s
build (CubeRedPrimary-bootloader, chibios, 10)	3m 17s
build (configure-all, chibios, 10)	12m 54s
build (build-options-defaults-test, chibios, 10)	10m 28s
build (signing, chibios, 10)	5m 7s
build (CubeOrange-PPP, chibios, 10)	4m 58s
build (CubeRed-EKF2, chibios, 10)	4m 31s
build (SOHW, chibios, 10)	5m 53s
build (Pixhawk6X-PPPGW, chibios, 10)	3m 33s
build (new-check, chibios, 10)	7m 29s
	2h 5m 55s

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