Documentation extended with specific information for VirtualBox

The configuration for EFI is enhanced to start U-Boot at VirtualBox (x86_64)
with all features like booting with help of boot scripts and extended linux.
This documentation describes how to use U-Boot at VirtualBox to boot
Linux systems with a some simple example.

Signed-off-by: Thomas Mittelstaedt <thomas.mittelstaedt@de.bosch.com>