When building for px4-v2 we have an warning because we are checking for
the value of this undefined macro. Just change both checks to use
"defined()".
../../libraries/AP_BoardConfig/AP_BoardConfig.cpp:36:7: warning: "CONFIG_ARCH_BOARD_PX4FMU_V4" is not defined [-Wundef]
#elif CONFIG_ARCH_BOARD_PX4FMU_V4
^