Makefile: use SPDX short identifier for Buildroot license

Signed-off-by: Rahul Bedarkar <rahulbedarkar89@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>

Makefile

@@ -736,7 +736,7 @@ legal-info-prepare: $(LEGAL_INFO_DIR)
 	@$(call legal-license-file,buildroot,COPYING,COPYING,HOST)
 	@$(call legal-manifest,PACKAGE,VERSION,LICENSE,LICENSE FILES,SOURCE ARCHIVE,SOURCE SITE,TARGET)
 	@$(call legal-manifest,PACKAGE,VERSION,LICENSE,LICENSE FILES,SOURCE ARCHIVE,SOURCE SITE,HOST)
-	@$(call legal-manifest,buildroot,$(BR2_VERSION_FULL),GPLv2+,COPYING,not saved,not saved,HOST)
+	@$(call legal-manifest,buildroot,$(BR2_VERSION_FULL),GPL-2.0+,COPYING,not saved,not saved,HOST)
 	@$(call legal-warning,the Buildroot source code has not been saved)
 	@cp $(BR2_CONFIG) $(LEGAL_INFO_DIR)/buildroot.config