bitbake: bb/utils.py: add break_hardlinks helper

bitbake: bb/utils.py: add break_hardlinks helper

bb.utils.copyfile is called in a few places with identical src and dst
in order to create an st_nlinks==1 version of the file. That that even
works relies on an implementation detail of copyfile (namely, that it
creates a temporary file and then does a rename). Moreover, it's a waste
of time if the file already has st_nlinks==1.

So create a helper that optimizes away the copy in the st_nlinks==1
case. Of course, this helper relies on the same implementation detail,
but that's now contained within bb.utils itself.

To test that we do at least sometimes hit the no-copy path, I tested
locally with

     if sstat[stat.ST_NLINK] == 1:
+        bb.note("Woohoo, 2*%d bytes I/O avoided" % sstat[stat.ST_SIZE])
         return True

(and the obvious places in oe-core patched), and the do_package log files
are indeed filled with woohoo notes.

(Bitbake rev: 7ae93cf40ab91965147055100432961436bce46c)

Signed-off-by: Rasmus Villemoes <rasmus.villemoes@prevas.dk>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>