leon3: add a little bootloader

This adds a little bootloader to the leon3_machine when a ram image is
given through the kernel parameter and no bios are provided:
  * The UART transmiter is enabled.
  * The TIMER is initialized.

Reviewed-by: Fabien Chouteau <chouteau@adacore.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Signed-off-by: KONRAD Frederic <frederic.konrad@adacore.com>
Signed-off-by: Mark Cave-Ayland <mark.cave-ayland@ilande.co.uk>