lib: share arm-selftest utility functions

arm-selftest has a couple utility functions that could be useful
to other unit tests, even other architectures. So move them out.
split_var moves to lib/util, where we can add other random
utilities over time. assert_args inspires report_abort, which
allows us to report a message, using the current prefix, that
we're aborting (outputs ABORT vs. PASS/FAIL). This is useful
for cases when unit tests can't complete due to missing
dependencies of some sort, such as missing/invalid inputs from
the user.

Signed-off-by: Andrew Jones <drjones@redhat.com>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Tested-by: Laurent Vivier <lvivier@redhat.com>
Reviewed-by: David Gibson <david@gibson.dropbear.id.au>