x86: Refresh subtargets kernel config

x86: Refresh subtargets kernel config

This was done by simply running `make kernel_menuconfig CONFIG_TARGET=subtarget`
and then saving without changing any option.

Having consistent kernel config is important to avoid surprises, such
as the issue fixed with 6f0367c9 (where Xen support was silently
disabled when building the kernel, although it was present in the
initial config)

As far as I understand the build system, this shouldn't have any
user-visible impact, because the build system already merges the
various kernel configs during build.

Signed-off-by: Baptiste Jonglez <git@bitsofnetworks.org>