classes/sanity: avoid bblayers version issue when switching between poky and other DISTRO values

If you create a build directory with poky, or set DISTRO to "poky" and
run bitbake, you'll get a bblayers.conf file with LCONF_VERSION set to
6. If you then set DISTRO to any other value where the new distro config
doesn't pull in poky's config, e.g. "nodistro", you would then get a
bblayers.conf version error which didn't immediately make sense. (The
layer versions have been out-of-step ever since meta-yocto-bsp was
split out of meta-yocto several years ago).

This is just painful and we'd rather users didn't have to deal with
it. Obviously it isn't an OE-Core problem per se, but a simple way to
resolve it for everyone is to bump OE-Core's version to 6 with an
automatic no-op upgrade. Also ensure that multiple upgrade functions
(such as the poky one) have a chance to execute by not breaking out of
the loop as we were before.

Fixes [YOCTO #6139].

(From OE-Core rev: 2fdeee2fad69445b0d97148826c7b027820be63a)

Signed-off-by: Paul Eggleton <paul.eggleton@linux.intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

meta/conf/sanity.conf

@@ -9,7 +9,7 @@ SANITY_ABIFILE = "${TMPDIR}/abi_version"
 
 SANITY_VERSION ?= "1"
 LOCALCONF_VERSION  ?= "1"
-LAYER_CONF_VERSION ?= "5"
+LAYER_CONF_VERSION ?= "6"
 SITE_CONF_VERSION  ?= "1"
 
 INHERIT += "sanity"