ROMFS: add dshot startup support

ROMFS/px4fmu_common/init.d/rcS

@@ -27,7 +27,6 @@ set FAILSAFE none
 set FAILSAFE_AUX none
 set FCONFIG /fs/microsd/etc/config.txt
 set FEXTRAS /fs/microsd/etc/extras.txt
-set FMU_ARGS ""
 set FMU_MODE pwm
 set FRC /fs/microsd/etc/rc.txt
 set IOFW "/etc/extras/px4_io-v2_default.bin"
@@ -521,7 +520,6 @@ unset FAILSAFE
 unset FAILSAFE_AUX
 unset FCONFIG
 unset FEXTRAS
-unset FMU_ARGS
 unset FMU_MODE
 unset FRC
 unset IO_PRESENT