[OE-core] TOPDIR and BB_HASHBASE_WHITELIST

[Mike Crowe]

Is it safe and/or sane to put TOPDIR in BB_HASHBASE_WHITELIST?

In my own recipes I make a few uses of TOPDIR and this is causing their
signatures to change between different trees. Simply adding it to
BB_HASHBASE_WHITELIST would avoid this problem but since TOPDIR is quite
fundamental I wondered if there might be some unwanted side effects.

In the past I've worked around this by introducing intermediate variables
and adding those to BB_HASHBASE_WHITELIST but it would be simpler to solve
the problem in one go.

Thanks.

Mike.