cfg80211: ship certificates as hex files

Not only does this remove the need for the hexdump code in most
normal kernel builds (still there for the extra directory), but
it also removes the need to ship binary files, which apparently
is somewhat problematic, as Randy reported.

While at it, also add the generated files to clean-files.

Reported-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>