vc_dispmanx_display_open: allow display to be set with VC_DISPLAY env variable

Allow end-users to set the default display through a VC_DISPLAY environement variable.
E.g. to force hello_teapot to display on HDMI one could execute:

VC_DISPLAY=5 ./hello_teapot.bin

Signed-off-by: Floris Bos <bos@je-eigen-domein.nl>

interface/vmcs_host/vc_vchi_dispmanx.c

@@ -25,6 +25,7 @@ ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
 SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 */
 #include <string.h>
+#include <stdlib.h>
 
 #include "vchost_config.h"
 #include "vchost.h"
@@ -510,6 +511,13 @@ VCHPRE_ int VCHPOST_ vc_dispmanx_resource_write_data_handle( DISPMANX_RESOURCE_H
  ***********************************************************/
 VCHPRE_ DISPMANX_DISPLAY_HANDLE_T VCHPOST_ vc_dispmanx_display_open( uint32_t device ) {
    uint32_t display_handle;
+   char *env = getenv("VC_DISPLAY");
+
+   if (device == 0 && env)
+   {
+      device = atoi(env);
+   }
+
    device = VC_HTOV32(device);
    display_handle = dispmanx_get_handle(EDispmanDisplayOpen,
                                                  &device, sizeof(device));