tools: replace private DOCDIR with automake docdir

Reuse automake variable instead of inventing a private variable.

The result of this command:
 git grep -wnl DOCDIR | xargs sed -i 's@DOCDIR@docdir@g'

Signed-off-by: Olaf Hering <olaf@aepfle.de>
Cc: Ian Jackson <ian.jackson@eu.citrix.com>
Cc: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
Cc: Ian Campbell <ian.campbell@citrix.com>
Cc: Wei Liu <wei.liu2@citrix.com>
Cc: Samuel Thibault <samuel.thibault@ens-lyon.org>
Acked-by: Ian Campbell <ian.campbell@citrix.com>

tools/xenmon/Makefile

@@ -31,8 +31,8 @@ install: build
 	$(INSTALL_PROG) xenbaked $(DESTDIR)$(sbindir)/xenbaked
 	$(INSTALL_PROG) xentrace_setmask  $(DESTDIR)$(sbindir)/xentrace_setmask
 	$(INSTALL_PROG) xenmon.py  $(DESTDIR)$(sbindir)/xenmon.py
-	$(INSTALL_DIR) $(DESTDIR)$(DOCDIR)
-	$(INSTALL_DATA) README $(DESTDIR)$(DOCDIR)/README.xenmon
+	$(INSTALL_DIR) $(DESTDIR)$(docdir)
+	$(INSTALL_DATA) README $(DESTDIR)$(docdir)/README.xenmon
 
 .PHONY: clean
 clean: