clean.mk: allow to forbid cleanup explicitly

The behaviour that sort of settled didn't actually follow
the principle of the least surprise when one really wanted
to have BUILDDIR available for inspection; DEBUG=2 would be
effective to achieve that but CLEAN=0 would not.

Thanks led@ for spotting and reporting this.

doc/params.txt

@@ -47,7 +47,7 @@
 ** экономия RAM+swap при сборке в tmpfs, иначе места на диске
 ** очистка рабочего каталога после успешной сборки очередной стадии
 ** может помешать использовать некоторые варианты отладки
-** значение: любая строка; по умолчанию пусто при DEBUG, иначе 1
+** значение: пусто, 0, 1, 2; по умолчанию пусто при DEBUG, иначе 1
 ** см. ../lib/clean.mk
 
 * DEBUG

lib/clean.mk

@@ -53,10 +53,14 @@ distclean: clean
 # than BUILDDIR variable is -- usually it's unneeded afterwards
 postclean: build-image
 	@{ \
-	if [ "0$(DEBUG)" -lt 2 -a -z "$(CHECK)$(REPORT)" ] && \
-	     [ "$(NUM_TARGETS)" -gt 1 \
+	if [ "$(CLEAN)" != 0 ] && \
+	   [ "0$(DEBUG)" -lt 2 ] && \
+	   [ -z "$(CHECK)" ] && \
+	   [ -z "$(REPORT)" ] && \
+	   [ "$(NUM_TARGETS)" -gt 1 \
 	       -o -n "$(__frontend)" \
-	       -o ! -L "$(SYMLINK)" ]; then \
+	       -o ! -L "$(SYMLINK)" ]; \
+	then \
 		echo "$(TIME) cleaning up after build"; \
 		$(MAKE) -C "$(BUILDDIR)" distclean \
 			GLOBAL_BUILDDIR="$(BUILDDIR)" $(LOG) ||:; \