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>

bitbake/lib/bb/data.py

@@ -297,6 +297,7 @@ def build_dependencies(key, keys, shelldeps, d):
             deps |= parser.references
             deps = deps | (keys & parser.execs)
         deps |= set((d.getVarFlag(key, "vardeps") or "").split())
+        deps -= set((d.getVarFlag(key, "vardepsexclude") or "").split())
     except:
         bb.note("Error expanding variable %s" % key) 
         raise