x86: add kmod-acpi-button and kmod-button-hotplug to default packages

Since r40909 openwrt can automatically shutdown when you press the
power button or try to shutdown a kvm virtual machine.
When booting a recent CC-rc2 or trunk image, it doesn't work,
as two kernel modules are missing.

Add these two kernel modules to the default packages, so users
don't need to install them manually.

Signed-off-by: Stefan Hellermann <stefan@the2masters.de>

SVN-Revision: 46366