env: Add 'envtools' target to 'make help'

The target is not currently mentioned anywhere. Add it to the help so
people can find it.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/Makefile b/Makefile
index 70971b1..5bb3b09 100644
--- a/Makefile
+++ b/Makefile
@@ -1586,6 +1586,7 @@
 	@echo  '  ubootrelease	  - Output the release version string (use with make -s)'
 	@echo  '  ubootversion	  - Output the version stored in Makefile (use with make -s)'
 	@echo  "  cfg		  - Don't build, just create the .cfg files"
+	@echo  "  envtools	  - Build only the target-side environment tools"
 	@echo  ''
 	@echo  'Static analysers'
 	@echo  '  checkstack      - Generate a list of stack hogs'