Use same PWM mapping for PPM and Serial RX

Without this fix an Octocopter with serialh RX will not work. Standard
 receiver mapping with only 6 motors outputs will apply.

src/main/drivers/pwm_mapping.c

@@ -28,6 +28,7 @@
 #include "pwm_output.h"
 #include "pwm_rx.h"
 #include "pwm_mapping.h"
+#include "config/config.h"
 /*
     Configuration maps
 
@@ -257,7 +258,7 @@ pwmOutputConfiguration_t *pwmInit(drv_pwm_config_t *init)
     // this is pretty hacky shit, but it will do for now. array of 4 config maps, [ multiPWM multiPPM airPWM airPPM ]
     if (init->airplane)
         i = 2; // switch to air hardware config
-    if (init->usePPM)
+    if (init->usePPM || init->useSerialRx)
         i++; // next index is for PPM
 
     setup = hardwareMaps[i];

src/main/drivers/pwm_mapping.h

@@ -38,6 +38,7 @@
 typedef struct drv_pwm_config_t {
     bool useParallelPWM;
     bool usePPM;
+    bool useSerialRx;
     bool useRSSIADC;
     bool useCurrentMeterADC;
 #ifdef STM32F10X

src/main/main.c

@@ -269,6 +269,7 @@ void init(void)
     pwm_params.useCurrentMeterADC = feature(FEATURE_CURRENT_METER);
     pwm_params.useLEDStrip = feature(FEATURE_LED_STRIP);
     pwm_params.usePPM = feature(FEATURE_RX_PPM);
+    pwm_params.useSerialRx = feature(FEATURE_RX_SERIAL);
     pwm_params.useServos = isMixerUsingServos();
     pwm_params.extraServos = currentProfile->gimbalConfig.gimbal_flags & GIMBAL_FORWARDAUX;
     pwm_params.motorPwmRate = masterConfig.motor_pwm_rate;