tools: fix "make distclean"

The original rule to target "distclean" in tools/Rules.mk was in effect
"make clean". It should be "make distclean".

However not all Makefiles in subdirectories have distclean target
defined. So this patch also adds a bunch of distclean targets to various
Makefiles. They only depend on clean target and don't have any actions
in most cases.

With the patch applied, following command outputs 0 results:

  find tools -name 'Makefile*' -exec grep -L 'distclean' {} \+ \
     | grep -v ocaml | grep -v libfsimage

Ocaml and libfsimage are known to have distclean defined in a dedicated
rules file.

Signed-off-by: Wei Liu <wei.liu2@citrix.com>
Cc: Ian Campbell <ian.campbell@citrix.com>
Cc: Ian Jackson <ian.jackson@eu.citrix.com>
Acked-by: Ian Jackson <ian.jackson@eu.citrix.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>

tools/ocaml/Makefile

@@ -20,3 +20,6 @@ install: subdirs-install
 
 .PHONY: clean
 clean: subdirs-clean
+
+.PHONY: distclean
+distclean: subdirs-distclean