Makefile: add list-defconfig to the general help

When listing defconfig files was moved to its own make target, it was
not added to the general help text.

However, this is a very important topic, so list it.

[Peter: drop ':' character]
Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Arnout Vandecappelle <arnout@mind.be>
Acked-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

Makefile

@@ -886,6 +886,7 @@ endif
 	@echo '  manual-epub            - build manual in ePub'
 	@echo '  graph-build            - generate graphs of the build times'
 	@echo '  graph-depends          - generate graph of the dependency tree'
+	@echo '  list-defconfigs        - list all defconfigs (pre-configured minimal systems)'
 	@echo
 	@echo 'Miscellaneous:'
 	@echo '  source                 - download all sources needed for offline-build'