selftests: fix build behaviour on targets' failures

Currently, when some of the KSFT subsystems fails to build, the toplevel
KSFT Makefile just keeps carrying on with the build process.

This behaviour is expected and desirable especially in the context of a CI
system running KSelfTest, since it is not always easy to guarantee that the
most recent and esoteric dependencies are respected across all KSFT TARGETS
in a timely manner.

Unfortunately, as of now, this holds true only if the very last of the
built subsystems could have been successfully compiled: if the last of
those subsystem instead failed to build, such failure is taken as the whole
outcome of the Makefile target and the complete build/install process halts
even though many other preceding subsytems were in fact already built
successfully.

Fix the KSFT Makefile behaviour related to all/install targets in order
to fail as a whole only when the all/install targets have failed for all
of the requested TARGETS, while succeeding when at least one of TARGETS
has been successfully built.

Signed-off-by: Cristian Marussi <cristian.marussi@arm.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>