param_export: reduce bson buffer size from 512 to 256

Some modules call this (and maybe in rare circumstances), which could lead
to stack overflows. With our current headroom of >= 300bytes, 256 should
be ok.

This increases the export time from ~28ms to ~34ms in my case (on a
Pixracer)

src/modules/systemlib/param/param.c

@@ -1048,7 +1048,7 @@ param_export(int fd, bool only_unsaved)
 
 	param_lock_reader();
 
-	uint8_t bson_buffer[512];
+	uint8_t bson_buffer[256];
 	bson_encoder_init_buf(&encoder, &bson_buffer, sizeof(bson_buffer));
 
 	/* no modified parameters -> we are done */

src/modules/systemlib/param/param.h

@@ -299,6 +299,7 @@ __EXPORT void		param_reset_excludes(const char *excludes[], int num_excludes);
 
 /**
  * Export changed parameters to a file.
+ * Note: this method requires a large amount of stack size!
  *
  * @param fd		File descriptor to export to.
  * @param only_unsaved	Only export changed parameters that have not yet been exported.
@@ -365,6 +366,7 @@ __EXPORT const char	*param_get_default_file(void);
 
 /**
  * Save parameters to the default file.
+ * Note: this method requires a large amount of stack size!
  *
  * This function saves all parameters with non-default values.
  *