Fixing bblayers conf versioning

This fix allows others consuming the meta-phosphor layer to update
bblayers conf independent of the upstream layer.

Change-Id: I8e170da62d260bbad175ed0fd13407886cb77e7f
Signed-off-by: Dave Cobbley <david.j.cobbley@linux.intel.com>
diff --git a/meta-phosphor/conf/distro/include/phosphor-base.inc b/meta-phosphor/conf/distro/include/phosphor-base.inc
index 1908118..d91dd7c 100644
--- a/meta-phosphor/conf/distro/include/phosphor-base.inc
+++ b/meta-phosphor/conf/distro/include/phosphor-base.inc
@@ -154,4 +154,4 @@
 # make sense on a BMC anyway.
 BAD_RECOMMENDATIONS_pn-obmc-phosphor-image = "udev-hwdb"
 
-LAYER_CONF_VERSION = "8"
+LAYER_CONF_VERSION ?= "8"