xen: arm: Select ramdisk bootmodule early enough when building dom0

xen: arm: Select ramdisk bootmodule early enough when building dom0

I thought in 0040b649d6df "xen: arm: Only lookup kernel/initrd bootmodule once
while building dom0" that I had identified place_modules as being soon enough,
but I was wrong, since write_properties wants to use it sooner.

Select the initrd boot module in kernel_probe at the same time we select the
kernel to avoid all this.

Signed-off-by: Ian Campbell <ian.campbell@citrix.com>