[oe] BB_STAMP_POLICY

[Koen Kooi]

FYI:

11:37:33 Koen: is BB_STAMP_POLICY documented somewhere?
11:37:47 Richard Purdie: Not really
11:38:03 Richard Purdie: Takes the values "file" "whitelist" or "full"
11:38:35 Richard Purdie: "file" is the traditional behaviour with stamps 
just checked within a given recipe
11:38:46 Richard Purdie: "full" checks all the dependencies are consistent
11:39:12 Richard Purdie: "whitelist" allows some packages to be excluded 
from the stamp checking basically so packages staging can work