tools: ipxe: Correct download error handling

This shell fragment lacked set -e.  So, eg if the download failed a
broken ipxe.tar.gz would be left behind.

Signed-off-by: Ian Jackson <ian.jackson@eu.citrix.com>
Reviewed-by: Paul Durrant <paul.durrant@citrix.com>
Tested-by: Paul Durrant <paul.durrant@citrix.com>
Acked-by: Wei Liu <wei.liu2@citrix.com>

tools/firmware/etherboot/Makefile

@@ -33,7 +33,7 @@ $(ROM): $(ROMS)
 	$(MAKE) -C $D/src bin/$(*F).rom
 
 $T:
-	if ! $(FETCHER) _$T $(IPXE_TARBALL_URL); then \
+	set -e; if ! $(FETCHER) _$T $(IPXE_TARBALL_URL); then \
 		$(GIT) clone $(IPXE_GIT_URL) $D.git; \
 		(cd $D.git && $(GIT) archive --format=tar --prefix=$D/ \
 		$(IPXE_GIT_TAG) | gzip -n >../_$T); \