HAL_PX4: writing more than 512 bytes at a time can cause IO errors

the vfat fs in NuttX is really very picky!

libraries/AP_HAL_PX4/Storage.cpp

@@ -28,7 +28,7 @@ void PX4Storage::_storage_create(void)
 {
 	mkdir(STORAGE_DIR, 0777);
 	unlink(STORAGE_FILE);
-	_fd = open(STORAGE_FILE, O_RDWR|O_CREAT|O_TRUNC, 0666);
+	_fd = open(STORAGE_FILE, O_RDWR|O_CREAT, 0666);
 	if (_fd == -1) {
 		hal.scheduler->panic("Failed to create " STORAGE_FILE);
 	}

libraries/AP_HAL_PX4/Storage.h

@@ -7,7 +7,7 @@
 #include "AP_HAL_PX4_Namespace.h"
 
 #define PX4_STORAGE_SIZE 4096
-#define PX4_STORAGE_MAX_WRITE 1024
+#define PX4_STORAGE_MAX_WRITE 512
 #define PX4_STORAGE_LINE_SHIFT 7
 #define PX4_STORAGE_LINE_SIZE (1<<PX4_STORAGE_LINE_SHIFT)
 #define PX4_STORAGE_NUM_LINES (PX4_STORAGE_SIZE/PX4_STORAGE_LINE_SIZE)