data_smart: Fix bug with overrides and weak default values

(aka pay the cookie monster for weak defaults)

If you have code like:

MYVAR = "a"
MYVAR_override ??= "b"

then MYVAR will get the value "a" even when override is in OVERRIDES. The
reason is that the value of ??= is set as a flag not a value and the cookie
monster isn't paid.

The fix is to ensure appropriate payment is made for a defaultval varflag
matching the usual setVar case.

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