selftests: allow detection of build failures

Commit 5f70bde2 ("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 <yauheni.kaliuta@redhat.com>
Signed-off-by: Jiri Benc <jbenc@redhat.com>
Reviewed-by: Cristian Marussi <cristian.marussi@arm.com>
Tested-by: Cristian Marussi <cristian.marussi@arm.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>