i386: express dependencies with Kconfig

This way, the default-configs file only need to specify the boards
and any optional devices.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Yang Zhong <yang.zhong@intel.com>
Message-Id: <20190123065618.3520-37-yang.zhong@intel.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>