kbuild: Add a help message for "headers"

Meanwhile explicitly state that the headers are uapi headers.

Suggested-by: Borislav Petkov <bp@alien8.de>
Signed-off-by: Xin Li (Intel) <xin@zytor.com>
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>

Author: xinli-intel

Makefile

@@ -1672,7 +1672,8 @@ help:
 	@echo  '  kernelrelease	  - Output the release version string (use with make -s)'
 	@echo  '  kernelversion	  - Output the version stored in Makefile (use with make -s)'
 	@echo  '  image_name	  - Output the image name (use with make -s)'
-	@echo  '  headers_install - Install sanitised kernel headers to INSTALL_HDR_PATH'; \
+	@echo  '  headers	  - Build ready-to-install UAPI headers in usr/include'
+	@echo  '  headers_install - Install sanitised kernel UAPI headers to INSTALL_HDR_PATH'; \
 	 echo  '                    (default: $(INSTALL_HDR_PATH))'; \
 	 echo  ''
 	@echo  'Static analysers:'