data_smart: Ensure variable flags are accounted for in config data hash

Currently if the flags set against a variable in the base data store
change, it doesn't automatically trigger a reparse when it really
should. For example with the blacklist class setting:

PNBLACKLIST[qemu] = "bar"
PNBLACKLIST[bash] = "foo"

will not trigger a reparse if only one entry is changed and a
blacklisted recipe can still be built.

I did consider using BB_SIGNATURE_EXCLUDE_FLAGS in here however it
doesn't make sense, we want to trigger a reparse when any of the
flags change too (which is different to the sstate signatures which
we wouldn't want to change in those cases).

[YOCTO #4627]

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