Impact: cleanup

Rather than overriding MODULE_PARAM_PREFIX, build via acpi.o so
KBUILD_MODNAME is set to "acpi".

This is the logical way to do it, even though acpi cannot be a module
due to these config options being bool.  Those parts of ACPI which can
be modular are not built into the acpi "module".
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
Signed-off-by: Len Brown <len.brown@intel.com>