bitbake.conf: define shared PERSISTENT_DIR

bitbake.conf: define shared PERSISTENT_DIR

* import from poky 6a11cf7dfe930461a6660e3b783b546fa2634900
* we had CACHE var pointing to machine specific dir since 2006
  caf077679022f37ce55d758101f130e4e93bd7b5
* current bitbake is looking for cache dir like this:
  cachedir = (bb.data.getVar("PERSISTENT_DIR", d, True) or
              bb.data.getVar("CACHE", d, True))
  which without PERSISTENT_DIR defined picks machine-specific CACHE dir
  so LOCALCOUNT numbers in SRCPV keeps rolling with every MACHINE switch
  in same builddir :/

Acked-by: Richard Purdie <richard.purdie@linuxfoundation.org>
Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>