build : provide FRU yaml as configurable input

In preparation for the openpower-vpd-parser recipe, make it possible to
provide the FRU data YAML file as a build-configurable input.

Use writefru.yaml as default.

Change-Id: I32dba99bcb6c8dd4da259f907b0499b3192cc412
Signed-off-by: Deepak Kodihalli <dkodihal@in.ibm.com>
diff --git a/Makefile.am b/Makefile.am
index 7ece020..8065a48 100644
--- a/Makefile.am
+++ b/Makefile.am
@@ -19,7 +19,7 @@
 	$(REQ_SCRIPT) \
 	$(REQ_SCRIPT_FILES)
 writefru.hpp: ${REQ_SCRIPT} ${REQ_SCRIPT_FILES}
-	$(AM_V_GEN)$(PYTHON) ${REQ_SCRIPT}
+	$(AM_V_GEN)@FRUGEN@
 
 sbin_PROGRAMS = openpower-read-vpd
 openpower_read_vpd_SOURCES = \