Ensure non-zero exit code on make failure

Before this change, we could have 0 exit code from the main
make process even when some targets failed to build on some
architectures:

$ make ARCH=mipsel [...] vm/tavolga-regular-mate-ex.recovery.tar
** ARCH: mipsel
10:15:06 cleaning up
10:15:06 initializing BUILDDIR: build/
10:15:06 preparing distro config: build/distcfg.mk
make[1]: *** No rule to make target 'use/systemd-coredump', needed by 'profile/.extra'.  Stop.
[nightly-mipsel@loongson5 mkimage-profiles]$ echo $?
0

Use `make -k' to get the old behaviour.

Makefile

@@ -66,6 +66,8 @@ SHELL = /bin/bash
 			if [ -n "$$REPORT" ]; then \
 				$(MAKE) -f reports.mk ARCH=$$ARCH; \
 			fi; \
+		else \
+			exit 1; \
 		fi; \
 	done; \
 	if [ "$$n" -lt "$(NUM_TARGETS)" ]; then say; fi