lib/clean.mk: post-build cleanup revisited

The prerequisites for a cleanup after a successful build
were somewhat weird at this point; now the rules are:
- if DEBUG level is more than 1 or CHECK is set, don't do it;
- otherwise if at least one of the following conditions is true:
  + there's more than one target being built in a row;
  + the build was run by e.g. alterator-mkimage;
  + metaprofile directory is read only
  ...then do a distclean.

If these are still weird or feel unsuitable for profile hacking,
drop me a note (or a patch).

doc/params.txt

@@ -41,7 +41,7 @@
 - CHECK
   + включает режим проверки сборки конфигурации (без сборки образа)
   + значение: пусто (по умолчанию) либо любая строка
-  + см. ../lib/build.mk
+  + см. ../lib/build.mk, ../lib/clean.mk
 
 - CLEAN
   + экономия RAM+swap при сборке в tmpfs, иначе места на диске
@@ -51,9 +51,9 @@
   + см. ../lib/clean.mk
 
 - DEBUG
-  + включает средства отладки
+  + включает средства отладки, может отключить зачистку после сборки
   + значение: пусто (по умолчанию), 1 или 2
-  + см. ../lib/log.mk
+  + см. ../lib/log.mk, ../lib/clean.mk
 
 - HOMEPAGE, HOMENAME, HOMEWAIT
   + указывают адрес, название и таймаут перехода для домашней страницы

lib/clean.mk

@@ -50,11 +50,15 @@ distclean: clean
 	} >&2
 
 # builddir existing outside read-only metaprofile is less ephemeral
-# than BUILDDIR is -- usually it's unneeded afterwards so just zap it
+# than BUILDDIR variable is -- usually it's unneeded afterwards
+# so just zap it unless DEBUG has been requested (moreso for multi
+# image builds, UI based ones, and readonly metaprofile; or CHECK)
 postclean: build-image
 	@{ \
-	if [ "$(NUM_TARGETS)" -gt 1 -a -z "$(DEBUG)" -a -z "$(__frontend)" ] ||\
-	    [ ! -L "$(SYMLINK)" -a "0$(DEBUG)" -lt 2 ]; then \
+	if [ "0$(DEBUG)" -lt 2 -a -z "$(CHECK)" ] && \
+	     [ "$(NUM_TARGETS)" -gt 1 \
+	       -o -n "$(__frontend)" \
+	       -o ! -L "$(SYMLINK)" ]; then \
 		echo "$(TIME) cleaning up after build"; \
 		$(MAKE) -C "$(BUILDDIR)" distclean \
 			GLOBAL_BUILDDIR="$(BUILDDIR)" $(LOG) ||:; \