Makefile: Let the 'help' target list the tools targets

List the name of the tool targets when calling 'make help':

  $ make help
  [...]
  Tools targets:
    qemu-ga                        - Build qemu-ga tool
    qemu-keymap                    - Build qemu-keymap tool
    elf2dmp                        - Build elf2dmp tool
    ivshmem-client                 - Build ivshmem-client tool
    ivshmem-server                 - Build ivshmem-server tool
    qemu-nbd                       - Build qemu-nbd tool
    qemu-img                       - Build qemu-img tool
    qemu-io                        - Build qemu-io tool
    qemu-edid                      - Build qemu-edid tool
    fsdev/virtfs-proxy-helper      - Build virtfs-proxy-helper tool
    scsi/qemu-pr-helper            - Build qemu-pr-helper tool

Signed-off-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

Makefile

@@ -1255,6 +1255,11 @@ endif
 		$(foreach t, $(TARGET_DIRS), \
 		$(call print-help-run,$(t)/all,Build for $(t));) \
 		echo '')
+	@$(if $(TOOLS), \
+		echo 'Tools targets:'; \
+		$(foreach t, $(TOOLS), \
+		$(call print-help-run,$(t),Build $(shell basename $(t)) tool);) \
+		echo '')
 	@echo  'Cleaning targets:'
 	$(call print-help,clean,Remove most generated files but keep the config)
 ifdef CONFIG_GCOV