gsm0808: Add create functions for CIPHER MODE COMMAND

gsm0808.h/c lacks functionality to generate CIPHER MODE COMMAND messages. These
messages are required if the code is used in an MSC implementation.

This commit adds a gsm0808_create_cipher() function, that generates an A/AoiP
CIPHER MODE COMMAND message.

Change-Id: I8eb1c357860c3e740b0f5d17e1c256bc87920958
diff --git a/tests/gsm0808/gsm0808_test.ok b/tests/gsm0808/gsm0808_test.ok
index f406551..8e2087d 100644
--- a/tests/gsm0808/gsm0808_test.ok
+++ b/tests/gsm0808/gsm0808_test.ok
@@ -4,6 +4,7 @@
 Testing creating Reset
 Testing creating Clear Command
 Testing creating Clear Complete
+Testing creating Chipher Mode Command
 Testing creating Cipher Complete
 Testing creating Cipher Reject
 Testing creating CM U