bitbake: bb.data_smart: only try to expand refs to valid variable names

bitbake: bb.data_smart: only try to expand refs to valid variable names

This aligns the behavior of expansion with the recipe parser, only
attempting to expand references to valid variable names. This avoids
adding references for things like `${foo#${TOPDIR}}` to our vardeps
without imposing much additional processing overhead beyond the change
to the expansion regexp.

YOCTO #12987

(Bitbake rev: df2ac65370aa86cdbc1574fdede25e3519410e45)

Signed-off-by: Christopher Larson <chris_larson@mentor.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>