data: add vardepvalueexclude varflag

On rare occasions it's useful to be able to exclude a part of a
variable's value from the variable's signature; for example if you want
to add an item to a list sometimes and not have the signature of the
variable change depending on whether the item is in the list or not. The
initial intended use case for this in OpenEmbedded is to allow adding a
function to SSTATEPOSTINSTFUNCS in buildhistory.bbclass and not have
that change any task signatures (so adding and removing
INHERIT += "buildhistory" won't lead to any rebuilds).

Part of the fix for [YOCTO #5897].

Signed-off-by: Paul Eggleton <paul.eggleton@linux.intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>