Include Qt recipes only when meta-qt5 layer is used

If the meta-qt5 layer is not included in the BBLAYERS, then do not
include our additional qt recipes from recipes-qt. This way meta-b2qt
can be used with meta-qt5 just to build the base images/toolchain.

By default, all bblayers.conf templates include meta-qt5, so it needs
to be remove first to use this feature.

Change-Id: I73b8f0e65a6f4f950498097a4801ba5f5325cd7d
Reviewed-by: Eirik Aavitsland <eirik.aavitsland@theqtcompany.com>

conf/layer.conf

@@ -24,9 +24,10 @@
 BBPATH .= ":${LAYERDIR}"
 
 # We have a recipes directory, add to BBFILES
-BBFILES += "${LAYERDIR}/recipes*/*/*.bb \
-            ${LAYERDIR}/recipes*/*/*.bbappend \
+BBFILES += "${LAYERDIR}/recipes/*/*.bb \
+            ${LAYERDIR}/recipes/*/*.bbappend \
             "
+BBFILES += "${@bb.utils.contains("BBFILE_COLLECTIONS", "qt5-layer", "${LAYERDIR}/recipes-qt/*/*.bb ${LAYERDIR}/recipes-qt/*/*.bbappend" , "", d)}"
 
 BBFILE_COLLECTIONS += "b2qt"
 BBFILE_PATTERN_b2qt := "^${LAYERDIR}/"