Makefile: enable dochelp run from main make level

Change to enable dochelp run from main make level to make it easier to
use it.
Signed-off-by: NShuah Khan <shuahkh@osg.samsung.com>
Signed-off-by: NJonathan Corbet <corbet@lwn.net>

Makefile

@@ -1454,7 +1454,7 @@ $(help-board-dirs): help-%:
 
 # Documentation targets
 # ---------------------------------------------------------------------------
-DOC_TARGETS := xmldocs latexdocs pdfdocs htmldocs epubdocs cleandocs linkcheckdocs
+DOC_TARGETS := xmldocs latexdocs pdfdocs htmldocs epubdocs cleandocs linkcheckdocs dochelp
 PHONY += $(DOC_TARGETS)
 $(DOC_TARGETS): scripts_basic FORCE
 	$(Q)$(MAKE) $(build)=Documentation $@