AP_BattMonitor: add option minimum volt option #932

Job	Run time
build (stm32f7, chibios, 10)	12m 43s
build (stm32h7, chibios, 10)	11m 6s
build (fmuv2-plane, chibios, 10)	6m 27s
build (periph-build, chibios, 10)	13m 5s
build (iofirmware, chibios, 10)	4m 15s
build (CubeOrange-bootloader, chibios, 10)	3m 1s
build (fmuv3-bootloader, chibios, 10)	3m 11s
build (revo-bootloader, chibios, 10)	3m 7s
build (stm32h7-debug, chibios, 10)	7m 40s
build (fmuv3, chibios, 10)	17m 27s
build (revo-mini, chibios, 10)	13m 8s
build (MatekF405-Wing, chibios, 10)	13m 42s
build (CubeOrange-ODID, chibios, 10)	9m 8s
build (CubeRedPrimary-bootloader, chibios, 10)	3m 12s
build (configure-all, chibios, 10)	12m 6s
build (build-options-defaults-test, chibios, 10)	19m 51s
build (signing, chibios, 10)	8m 59s
build (CubeOrange-PPP, chibios, 10)	7m 33s
build (CubeOrange-SOHW, chibios, 10)	6m 56s
build (Pixhawk6X-PPPGW, chibios, 10)	4m 33s
	3h 1m 10s

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