AP_BattMonitor: add option minimum volt option #926

Job	Run time
build (stm32f7, chibios, 10)	2m 12s
build (stm32h7, chibios, 10)	1m 58s
build (fmuv2-plane, chibios, 10)	2m 1s
build (periph-build, chibios, 10)	2m 19s
build (iofirmware, chibios, 10)	3m 12s
build (CubeOrange-bootloader, chibios, 10)	3m 7s
build (fmuv3-bootloader, chibios, 10)	3m 13s
build (revo-bootloader, chibios, 10)	3m 11s
build (stm32h7-debug, chibios, 10)	2m 0s
build (fmuv3, chibios, 10)	3m 13s
build (revo-mini, chibios, 10)	2m 49s
build (MatekF405-Wing, chibios, 10)	3m 11s
build (CubeOrange-ODID, chibios, 10)	2m 6s
build (CubeRedPrimary-bootloader, chibios, 10)	3m 14s
build (configure-all, chibios, 10)	3m 14s
build (build-options-defaults-test, chibios, 10)	2m 6s
build (signing, chibios, 10)	2m 8s
build (CubeOrange-PPP, chibios, 10)	2m 18s
build (CubeOrange-SOHW, chibios, 10)	2m 7s
build (Pixhawk6X-PPPGW, chibios, 10)	1m 26s
	51m 5s

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