zephyr: Move testplan into sample Makefile

Instead of a bunch of patches that tend to become conflicting, use the
newly parameterized Makefile to make all of the test plans into make
targets.  Update the instructions to match this.

Signed-off-by: David Brown <david.brown@linaro.org>
diff --git a/samples/zephyr/bad-keys/bad-ec-p256.pem b/samples/zephyr/bad-keys/bad-ec-p256.pem
new file mode 100644
index 0000000..333f41f
--- /dev/null
+++ b/samples/zephyr/bad-keys/bad-ec-p256.pem
@@ -0,0 +1,5 @@
+-----BEGIN EC PRIVATE KEY-----
+MHcCAQEEILmqmiH6y3EGhLkTcnNtU7hZ1wnc51MIL53npseRX7vJoAoGCCqGSM49
+AwEHoUQDQgAEcX9ExNjZfsckp6AdutjPjVJsvP6ZZkKfLsGnRpKR+9OpO9/qmJHs
+ks+ZXo70SEANjWnNlxKNAVci8aUm8UskLw==
+-----END EC PRIVATE KEY-----