sh function instead of alias for op-build

Signed-off-by: Stewart Smith <stewart@linux.vnet.ibm.com>
diff --git a/op-build-env b/op-build-env
index 81cf898..9cd5cb5 100755
--- a/op-build-env
+++ b/op-build-env
@@ -8,5 +8,6 @@
 export BR2_EXTERNAL=${__PWD}/openpower
 export BR2_DL_DIR=${__PWD}/dl
 
-shopt -s expand_aliases
-alias op-build="make --directory=${__PWD}/buildroot O=${__PWD}/output "
+op-build () {
+    make --directory=${__PWD}/buildroot O=${__PWD}/output $@
+}