Makefile: Add manual target to build user manual

Change-Id: I61ff89db5666248de47cb9752a2aecdd6fd06f02
diff --git a/Makefile b/Makefile
index eb2f8d8..ee6300e 100644
--- a/Makefile
+++ b/Makefile
@@ -1,4 +1,4 @@
-all: deps version check
+all: deps version check manual
 
 .PHONY: version check
 
@@ -8,8 +8,11 @@
 version:
 	./update_version.sh
 
+manual:
+	$(MAKE) -C doc/manuals
+
 check:
-	$(MAKE) -C selftest check	
+	$(MAKE) -C selftest check
 	@echo "make check: success"
 
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