build: fix device module builds

Slightly hackish workaround, works ok as long as we don't
have target-specific modules.  meson will obsolete this.

See comment in the patch for the --verbose description.

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Message-id: 20200624131045.14512-5-kraxel@redhat.com

[ kraxel: updated comment from discussions ]

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>

Makefile.target

@@ -179,6 +179,20 @@ endif # CONFIG_SOFTMMU
 dummy := $(call unnest-vars,,obj-y)
 all-obj-y := $(obj-y)
 
+#
+# common-obj-m has some crap here, probably as side effect from
+# unnest-vars recursing into target directories to fill obj-y and not
+# properly handling the -m case.
+#
+# Clear common-obj-m as workaround.  Fixes suspious dependency errors
+# when building devices as modules.  A bit hackish, but should be ok
+# as long as we do not have any target-specific modules.
+#
+# The meson-based build system currently in development doesn't need
+# unnest-vars and will obsolete this workaround.
+#
+common-obj-m :=
+
 include $(SRC_PATH)/Makefile.objs
 dummy := $(call unnest-vars,.., \
                authz-obj-y \