configs: add support for Synopsys VDK Software Development Platform

The Virtualizer Development Kit (VDK) Family for ARM Cortex Products
consists of a set of virtual prototypes that provide a virtualizer for
the ARM core variants. The VDK is a standalone package that runs on an
host computer.

[Thomas: drop the following options, which as noted by Arnout, are not
needed:
 - BR2_LINUX_KERNEL_IMAGE_TARGET_CUSTOM=y
 - BR2_LINUX_KERNEL_IMAGE_TARGET_NAME="Image"
 - BR2_PACKAGE_BUSYBOX_SHOW_OTHERS=y.]

Signed-off-by: Joao Pinto <jpinto@synopsys.com>
Reviewed-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>