help.mk, clean.mk: drop __frontend for tty test

The __frontend variable was introduced to address the needs
of alterator-mkimage module: list the images available in
one column, purge the builddir.

Looks like we should consider other cases with redirected
stdout (cron builds, piped calls, etc) like fundamentally
non-interactive and behave the same.

So commit 3a8af6b55d888d25c1d97561ed2ecf37ff28ad71's
description is wrong now; the current cleanup rules are:

- if CLEAN=0 or DEBUG>1, don't do it;
- if CHECK or REPORT 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;
  + stdout was redirected (cronjob, alterator-mkimage...);
  + metaprofile directory is read-only
  ...then do a distclean.

If that doesn't suit your needs, describe the particular
situation please.

Thanks cas@ for wondering aloud whether greppable output
is unsupported with `make help'.

lib/clean.mk

@@ -58,7 +58,7 @@ postclean: build-image
 	   [ -z "$(CHECK)" ] && \
 	   [ -z "$(REPORT)" ] && \
 	   [ "$(NUM_TARGETS)" -gt 1 \
-	       -o -n "$(__frontend)" \
+	       -o ! -t 1 \
 	       -o ! -L "$(SYMLINK)" ]; \
 	then \
 		echo "$(TIME) cleaning up after build"; \

lib/help.mk

@@ -1,15 +1,13 @@
 # this makefile holds the most helpful bits for the toplevel one
 
-ifdef __frontend
-define help_body
-	@for i in $(2); do echo $$i; done
-endef
-else
 define help_body
+if [ -t 1 ]; then \
 	echo '** available $(1) targets:'; \
-	columnize $(2)
+	columnize $(2); \
+else \
+	printf '%s\n' $(2); \
+fi
 endef
-endif
 
 help = $(and $(2),$(help_body))