package.bbclass: Add CONFFILES to list of package specific variables

Changes to CONFFILES should change the sstate checksum. To make that happen,
it needs to be listed in the list of package specific variables, therefore
add it.

(From OE-Core rev: 9db71fa03b9d5f5307b2d09e7aa89f46f622aa09)

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>