Upload kernel-devel package built for dom0 to VM repository

When VM kernel is provided by dom0 (the default in Qubes 3.x), VM
kernel-devel package should match kernel actually installed in dom0.

Fixes QubesOS/qubes-issues#2641

(cherry picked from commit d3c09e68ad)

Makefile.builder

@@ -1,8 +1,14 @@
 ifeq ($(PACKAGE_SET),dom0)
 RPM_SPEC_FILES := kernel.spec
 else ifeq ($(PACKAGE_SET),vm)
+
 ifdef UPDATE_REPO
+# If DIST_DOM0 defined, copy kernel-devel from there
+ifneq ($(DIST_DOM0),)
 # Include kernel-devel packages in VM repo - dummy spec file
 RPM_SPEC_FILES := kernel-devel.spec
+OUTPUT_DIR = pkgs/$(DIST_DOM0)
 endif
 endif
+
+endif