makefile: change build order - build core-vm package first

Build core-vm package before doc and kernel-placeholder. If compile
core-vm would fail, it will fail earlier, which improve developement
efficiency.

Makefile.builder

@@ -1,4 +1,6 @@
 ifeq ($(PACKAGE_SET),vm)
-RPM_SPEC_FILES := rpm_spec/core-vm-doc.spec rpm_spec/core-vm-kernel-placeholder.spec rpm_spec/core-vm.spec
+RPM_SPEC_FILES := rpm_spec/core-vm.spec \
+    rpm_spec/core-vm-doc.spec \
+    rpm_spec/core-vm-kernel-placeholder.spec
 ARCH_BUILD_DIRS := archlinux
 endif