Makefile: fix tags target documentation

Replace the TAGS target name by the actual ctags target name.
Also, add etags target documentation.

Cc: Masahiro Yamada <yamada.m@jp.panasonic.com>
Signed-off-by: Igor Grinberg <grinberg@compulab.co.il>

Makefile

@@ -1311,7 +1311,8 @@ help:
 	@echo  '  dir/file.[oisS] - Build specified target only'
 	@echo  '  dir/file.lst    - Build specified mixed source/assembly target only'
 	@echo  '                    (requires a recent binutils and recent build (System.map))'
-	@echo  '  tags/TAGS	  - Generate tags file for editors'
+	@echo  '  tags/ctags	  - Generate ctags file for editors'
+	@echo  '  etags		  - Generate etags file for editors'
 	@echo  '  cscope	  - Generate cscope index'
 	@echo  '  ubootrelease	  - Output the release version string'
 	@echo  '  ubootversion	  - Output the version stored in Makefile'