Add openPOWER version string support

See VERSION.readme for more info
diff --git a/op-build-env b/op-build-env
index 79299c8..9f2bfd2 100755
--- a/op-build-env
+++ b/op-build-env
@@ -1,6 +1,10 @@
 #!/bin/bash
 __PWD=`pwd`
 
+if [ -e ./customrc ]; then
+    source ./customrc
+fi
+
 export BR2_EXTERNAL=${__PWD}/openpower
 export BR2_DL_DIR=${__PWD}/dl