AVAILTUNES is only used as a sanity check, we don't need to
include it in the sstate checksum in this case. If included
it can cause problems when switching machines with a common
package architecture.
[YOCTO #3667]
(From OE-Core rev: b1cc5a4055c6402595eac1a93eac4c946210d130)
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>