selftests: allow detection of build failures

Commit 5f70bde26a48 ("selftests: fix build behaviour on targets' failures")
added a logic to track failure of builds of individual targets. However, it
does exactly the opposite of what a distro kernel needs: we create a RPM
package with a selected set of selftests and we need the build to fail if
build of any of the targets fail.

Both use cases are valid. A distribution kernel is in control of what is
included in the kernel and what is being built; any error needs to be
flagged and acted upon. A CI system that tries to build as many tests as
possible on the best effort basis is not really interested in a failure here
and there.

Support both use cases by introducing a FORCE_TARGETS variable. It is
switched off by default to make life for CI systems easier, distributions
can easily switch it on while building their packages.

Reported-by: Yauheni Kaliuta <[e-mail]>
Signed-off-by: Jiri Benc <[e-mail]>
Reviewed-by: Cristian Marussi <[e-mail]>
Tested-by: Cristian Marussi <[e-mail]>
Signed-off-by: Shuah Khan <[e-mail]>

tools/testing/selftests/Makefile

···
 	override TARGETS := $(TMP)
 endif
 
+# User can set FORCE_TARGETS to 1 to require all targets to be successfully
+# built; make will fail if any of the targets cannot be built. If
+# FORCE_TARGETS is not set (the default), make will succeed if at least one
+# of the targets gets built.
+FORCE_TARGETS ?=
+
 # Clear LDFLAGS and MAKEFLAGS if called from main
 # Makefile to avoid test build failures when test
 # Makefile doesn't have explicit build rules.
···
 	for TARGET in $(TARGETS); do				\
 		BUILD_TARGET=$$BUILD/$$TARGET;			\
 		mkdir $$BUILD_TARGET  -p;			\
-		$(MAKE) OUTPUT=$$BUILD_TARGET -C $$TARGET;	\
+		$(MAKE) OUTPUT=$$BUILD_TARGET -C $$TARGET	\
+				$(if $(FORCE_TARGETS),|| exit);	\
 		ret=$$((ret * $$?));				\
 	done; exit $$ret;
 
···
 	@ret=1;	\
 	for TARGET in $(TARGETS); do \
 		BUILD_TARGET=$$BUILD/$$TARGET;	\
-		$(MAKE) OUTPUT=$$BUILD_TARGET -C $$TARGET INSTALL_PATH=$(INSTALL_PATH)/$$TARGET install; \
+		$(MAKE) OUTPUT=$$BUILD_TARGET -C $$TARGET INSTALL_PATH=$(INSTALL_PATH)/$$TARGET install \
+				$(if $(FORCE_TARGETS),|| exit);	\
 		ret=$$((ret * $$?));		\
 	done; exit $$ret;