templates/20_linux_xen: Ignore xenpolicy and config files too

templates/20_linux_xen: Ignore xenpolicy and config files too

file_is_not_sym() currently only checks for xen-syms. Extend it to
disregard xenpolicy (XSM policy files) and files ending .config (which
are built by the Xen upstream build system in some configurations and
can therefore end up in /boot).

Rename the function accordingly, to file_is_not_xen_garbage().

Signed-off-by: Ian Jackson <ian.jackson@eu.citrix.com>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>