Create virtual packages for OpenPower toolchain

Create a "toolchain" category, plus "p8-pore-toolchain" and
"ppe42-toolchain" virtual-packages, with the goal of lowering the
coupling between these toolchain and target packages.

Signed-off-by: Klaus Heinrich Kiwi <klaus@linux.vnet.ibm.com>
diff --git a/openpower/package/occ/Config.in b/openpower/package/occ/Config.in
index d9df0b3..d3be09d 100644
--- a/openpower/package/occ/Config.in
+++ b/openpower/package/occ/Config.in
@@ -1,6 +1,7 @@
 config BR2_PACKAGE_OCC
         bool "OCC (POWER9)"
         default y if (BR2_OPENPOWER_PLATFORM && BR2_OPENPOWER_POWER9)
+        depends on BR2_PACKAGE_HAS_PPE42_TOOLCHAIN
         select BR2_CPP
         help
             Project to build OCC