lib/build.mk: tweaked wreckage filter

A pretty common issue breaking the image build is inter-package
file conflict resulting in hsh-install failure down there.
Let's bring that back to attention conveniently.

lib/build.mk

@@ -2,6 +2,8 @@
 ANSI_OK   ?= 1;32
 ANSI_FAIL ?= 1;31
 
+GOTCHA := ^(((\*\* )?(E:|[Ee]rror|[Ww]arning).*)|(.* (Stop|failed)\.))$$
+
 ifndef MKIMAGE_PROFILES
 $(error this makefile is designed to be included in toplevel one)
 endif
@@ -48,7 +50,7 @@ build-image: profile/populate
 		echo "$(TIME) done (`tail -1 $(BUILDLOG) | cut -f1 -d.`)"; \
 		tail -200 "$(BUILDLOG)" $(SHORTEN) \
 		| GREP_COLOR="$(ANSI_OK)" GREP_OPTIONS="--color=auto" \
-		  grep '^\*\* image: .*' ||:; \
+		  grep '^\*\* image: .*$$' ||:; \
 	else \
 		echo -n "$(TIME) failed, see log"; \
 		if [ -z "$(DEBUG)" ]; then \
@@ -59,7 +61,7 @@ build-image: profile/populate
 		fi; \
 		tail -200 "$(BUILDLOG)" \
 		| GREP_COLOR="$(ANSI_FAIL)" GREP_OPTIONS="--color=auto" \
-		  egrep "^((E:|[Ee]rror|[Ww]arning).*)|(.* Stop\.)$$"; \
+		  egrep "$(GOTCHA)"; \
 		df -P $(BUILDDIR) | awk 'END { if ($$4 < $(LOWSPACE)) \
 			{ print "NB: low space on "$$6" ("$$5" used)"}}'; \
 	fi; \