Kconfig: Introduce CONFIG_XEN

Introduce CONFIG_XEN to make U-Boot could be used as bootloader
for a virtual machine.

Without bootloader, we could successfully boot up android on XEN, but
we need need bootloader to support A/B, dm verify and etc.

Signed-off-by: Peng Fan <peng.fan@nxp.com>
Signed-off-by: Oleksandr Andrushchenko <oleksandr_andrushchenko@epam.com>
Signed-off-by: Anastasiia Lukianenko <anastasiia_lukianenko@epam.com>