AP_BoardConfig: fix check for PX4_V4

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
       ^

libraries/AP_BoardConfig/AP_BoardConfig.cpp

@@ -30,10 +30,10 @@
 #include <drivers/drv_pwm_output.h>
 #include <drivers/drv_sbus.h>
 
-#ifdef CONFIG_ARCH_BOARD_PX4FMU_V1
+#if defined(CONFIG_ARCH_BOARD_PX4FMU_V1)
 #define BOARD_PWM_COUNT_DEFAULT 2
 #define BOARD_SER1_RTSCTS_DEFAULT 0 // no flow control on UART5 on FMUv1
-#elif CONFIG_ARCH_BOARD_PX4FMU_V4
+#elif defined(CONFIG_ARCH_BOARD_PX4FMU_V4)
 #define BOARD_PWM_COUNT_DEFAULT 6
 #define BOARD_SER1_RTSCTS_DEFAULT 2
 #else // V2