PX4 System:Expunge the nuttx adc structure from the system

This PR is preliminary ground work for FMUv5.

   PX4 does not use the NuttX adc driver. But used the same format
   for the data returned by the nuttx ADC driver.

   There was a fixme:in src/platforms/px4_adc.h "this needs to be
   a px4_adc_msg_s type" With this PR the need for
   src/platforms/px4_adc.h goes away as the driver drv_adc.h now
   describes the px4_adc_msg_t.

src/platforms/posix/drivers/adcsim/adcsim.cpp

@@ -42,7 +42,6 @@
 
 #include <px4_config.h>
 #include <px4_time.h>
-#include <px4_adc.h>
 #include <board_config.h>
 #include <drivers/device/device.h>
 
@@ -84,7 +83,7 @@ private:
 	perf_counter_t		_sample_perf;
 
 	unsigned		_channel_count;
-	adc_msg_s		*_samples;		/**< sample buffer */
+	px4_adc_msg_t	*_samples;		/**< sample buffer */
 
 	/** worker function */
 	virtual void		_measure();
@@ -119,7 +118,7 @@ ADCSIM::ADCSIM(uint32_t channels) :
 		}
 	}
 
-	_samples = new adc_msg_s[_channel_count];
+	_samples = new px4_adc_msg_t[_channel_count];
 
 	/* prefill the channel numbers in the sample array */
 	if (_samples != nullptr) {
@@ -145,7 +144,7 @@ ADCSIM::~ADCSIM()
 ssize_t
 ADCSIM::devRead(void *buffer, size_t len)
 {
-	const size_t maxsize = sizeof(adc_msg_s) * _channel_count;
+	const size_t maxsize = sizeof(px4_adc_msg_t) * _channel_count;
 
 	if (len > maxsize) {
 		len = maxsize;
@@ -204,7 +203,7 @@ test()
 	}
 
 	for (unsigned i = 0; i < 50; i++) {
-		adc_msg_s data[12];
+		px4_adc_msg_t data[PX4_MAX_ADC_CHANNELS];
 		ssize_t count = h.read(data, sizeof(data));
 
 		if (count < 0) {

src/platforms/posix/include/system_config.h

@@ -39,6 +39,11 @@
 
 #pragma once
 
+#define ADC_BATTERY_VOLTAGE_CHANNEL     10
+#define ADC_BATTERY_CURRENT_CHANNEL     ((uint8_t)(-1))
+#define ADC_AIRSPEED_VOLTAGE_CHANNEL    11
+#define ADCSIM0_DEVICE_PATH	"/dev/adc0"
+
 /*
  * I2C busses
  */