manual generation: update help

The 'make help' lines for generating the manual did not consider ePub
correctly.  Instead of providing a limitative list for 'make manual', the
text is changed into 'all formats'.

Signed-off-by: Thomas De Schampheleire <thomas.de.schampheleire@gmail.com>
Acked-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>

Makefile

@@ -811,7 +811,7 @@ ifeq ($(BR2_TARGET_BAREBOX),y)
 endif
 	@echo
 	@echo 'Documentation:'
-	@echo '  manual                 - build manual in HTML, split HTML, PDF and txt'
+	@echo '  manual                 - build manual in all formats'
 	@echo '  manual-html            - build manual in HTML'
 	@echo '  manual-split-html      - build manual in split HTML'
 	@echo '  manual-pdf             - build manual in PDF'