Added bootloaders to the recipe of every machine

 * Include file with
 * EXTRA_IMAGEDEPENDS += "at91bootstrap u-boot"

Signed-off-by: Marco Cavallini <m.cavallini@koansoftware.com>
Unified usba_udc and g_serial setting for all atmel-xplained-*-image.bb

 * Moved common parts in atmel-demo-image.inc

Signed-off-by: Marco Cavallini <m.cavallini@koansoftware.com>