kselftest: add capability to skip chosen TARGETS

Let the user specify an optional TARGETS skiplist through the new optional
SKIP_TARGETS Makefile variable.

It is easier to skip at will using a reduced and well defined list of
possibly problematic targets with SKIP_TARGETS than to provide a partially
stripped down list of good targets using the usual TARGETS variable.

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