src/poky:bitbake: data_smart: Small optimisation to _findVar()

Some users of _findVar don't need the override data and even
getVarFlag doesn't need it in some common cases (parsing=True).

Rearrange the code as the current overridedata approach doesn't need
to be in the _findVar code anyway. This removes some search overhead
from a critical path.

(Bitbake rev: fcb64e1138a20eb19560af3fc5d1fa748cc9cf34)

Signed-off-by: Richard Purdie <[email protected]>