bitbake: allow excluding dependencies of variables

bitbake: allow excluding dependencies of variables

Adds a vardepsexclude flag that can be used to exclude a dependency of
a variable (the opposite of vardeps). This will allow the exclusion of
variables from the hash generation much more selectively than blanket
whitelisting using BB_HASHBASE_WHITELIST.

Signed-off-by: Paul Eggleton <paul.eggleton@linux.intel.com>