HAL_Linux: split RCInput class into generic and PRU implementations

this will make it easier for other boards (such as NAVIO) to implement
their own RCInput mechanism

libraries/AP_HAL_Linux/RCInput_PRU.cpp

@@ -0,0 +1,59 @@
+#include <AP_HAL.h>
+
+#if CONFIG_HAL_BOARD == HAL_BOARD_LINUX
+#include <stdio.h>
+#include <sys/time.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+#include <errno.h>
+#include <unistd.h>
+#include <fcntl.h>
+#include <poll.h>
+#include <sys/mman.h>
+#include <sys/stat.h>
+#include <stdint.h>
+
+#include "RCInput.h"
+
+extern const AP_HAL::HAL& hal;
+
+using namespace Linux;
+
+void LinuxRCInput_PRU::init(void*)
+{
+    int mem_fd = open("/dev/mem", O_RDWR|O_SYNC);
+    if (mem_fd == -1) {
+        hal.scheduler->panic("Unable to open /dev/mem");
+    }
+    ring_buffer = (volatile struct ring_buffer*) mmap(0, 0x1000, PROT_READ|PROT_WRITE, 
+                                                      MAP_SHARED, mem_fd, RCIN_PRUSS_SHAREDRAM_BASE);
+    close(mem_fd);
+    ring_buffer->ring_head = 0;
+    _s0_time = 0;
+}
+
+/*
+  called at 1kHz to check for new pulse capture data from the PRU
+ */
+void LinuxRCInput_PRU::_timer_tick()
+{
+    while (ring_buffer->ring_head != ring_buffer->ring_tail) {
+        if (ring_buffer->ring_tail >= NUM_RING_ENTRIES) {
+            // invalid ring_tail from PRU - ignore RC input
+            return;
+        }
+        if (ring_buffer->buffer[ring_buffer->ring_head].pin_value == 1) {
+            // remember the time we spent in the low state
+            _s0_time = ring_buffer->buffer[ring_buffer->ring_head].delta_t;
+        } else {
+            // the pulse value is the sum of the time spent in the low
+            // and high states
+            _process_ppmsum_pulse(ring_buffer->buffer[ring_buffer->ring_head].delta_t + _s0_time);
+        }
+        // move to the next ring buffer entry
+        ring_buffer->ring_head = (ring_buffer->ring_head + 1) % NUM_RING_ENTRIES;        
+    }
+}
+
+#endif // CONFIG_HAL_BOARD