Use op-build generated SKIBOOT_VERSION_FILE when building skiboot

Signed-off-by: Stewart Smith <stewart@linux.vnet.ibm.com>
diff --git a/openpower/package/skiboot/skiboot.mk b/openpower/package/skiboot/skiboot.mk
index c4e1c0b..ef2798c 100644
--- a/openpower/package/skiboot/skiboot.mk
+++ b/openpower/package/skiboot/skiboot.mk
@@ -26,7 +26,7 @@
 endif
 
 define SKIBOOT_BUILD_CMDS
-	$(TARGET_CONFIGURE_OPTS) SKIBOOT_VERSION=$(SKIBOOT_VERSION) \
+	$(TARGET_CONFIGURE_OPTS) SKIBOOT_VERSION=`cat $(SKIBOOT_VERSION_FILE)` \
 		$(MAKE) $(SKIBOOT_MAKE_OPTS) -C $(@D) all
 endef