goldfish: refactor goldfish platform configs

On new virtual devices, the goldfish virtual bus can be replaced with
autoprobing infrastructure like Device Tree.  Refactor the goldfish
kernel configs to better accommodate this.

Move the goldfish platform into a menuconfig in the style of the chrome
platform, and separate the goldfish bus into its own config option.

Signed-off-by: Greg Hackmann <ghackmann@google.com>
Signed-off-by: Jin Qian <jinqian@android.com>
[Corrected a tristate to bool]
Signed-off-by: Alan Cox <alan@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>