selftests/run_kselftest.sh: Use readlink if realpath is not available

'realpath' is not always available, fallback to 'readlink -f' if is not
available. They seem to work equally well in this context.

Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Yosry Ahmed <[e-mail]>
Signed-off-by: Shuah Khan <[e-mail]>

tools/testing/selftests/run_kselftest.sh

···
 #
 # Run installed kselftest tests.
 #
-BASE_DIR=$(realpath $(dirname $0))
+
+# Fallback to readlink if realpath is not available
+if which realpath > /dev/null; then
+        BASE_DIR=$(realpath $(dirname $0))
+else
+        BASE_DIR=$(readlink -f $(dirname $0))
+fi
+
 cd $BASE_DIR
 TESTS="$BASE_DIR"/kselftest-list.txt
 if [ ! -r "$TESTS" ] ; then