iso.mk: low space hint on build failure

Just in case the build.log will be inobvious, and it's easy to diagnose
automatically.  Thanks Andrey Stroganov for this use case.

Thanks for improving the initial implementation go to raorn@ for kind
commit lynch and to Yuri Bushmelev for actually suggesting something
more concise.

BTW the "1024" magic number was taken out of thin air:
the "no free space" errors are most likely to happen while
forming/populating a chroot (apt/rpm errors out) and chroots are
roughly two orders of magnitude heftier than a megabyte.

bin/mktmpdir

@@ -26,4 +26,4 @@ choose_tmpdir() {
 }
 
 DIR="`choose_tmpdir`"
-mktemp -d "${1:-tmpdir}.XXXXXXXXXX" --tmpdir="${DIR:-..}"
+mktemp -d "${1:-tmpdir}.XXXXXXXXXX" --tmpdir="${DIR:-`realpath ..`}"

iso.mk

@@ -14,7 +14,7 @@ export ARCH ?= $(shell arch | sed 's/i686/i586/')
 
 iso:
 	@echo -n "** starting image build"
-	@if test -n "$(DEBUG)"; then \
+	@if [ -n "$(DEBUG)" ]; then \
 		echo ": tail -f $(BUILDLOG)" $(SHORTEN); \
 	else \
 		echo " (coffee time)"; \
@@ -26,8 +26,10 @@ iso:
 			|| echo "no log"`)"; \
 	else \
 		echo "** build failed, see log: $(BUILDLOG)" $(SHORTEN); \
-		if test -z "$(DEBUG)"; then \
+		if [ -z "$(DEBUG)" ]; then \
 			echo "   (you might want to re-run with DEBUG=1)"; \
 		fi; \
 		tail -100 "$(BUILDLOG)" | egrep "^E:|[Ee]rror|[Ww]arning"; \
+		df -P $(BUILDDIR) | awk 'END { if ($$4 < 1024) \
+			{ print "** NB: low space on "$$6" ("$$5" used)"}}'; \
 	fi