gsm0808: Add create functions for BSS_MAP_MSG_ASSIGMENT_RQST

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

This commit adds a gsm0808_create_assignment() function, that generates an
A/AoiP BSS_MAP_MSG_PAGING message.

Change-Id: I4d1d455a1e1cf95407e23ded7b7defbcf2dd6ff0
diff --git a/tests/gsm0808/gsm0808_test.ok b/tests/gsm0808/gsm0808_test.ok
index 6170a7a..52af134 100644
--- a/tests/gsm0808/gsm0808_test.ok
+++ b/tests/gsm0808/gsm0808_test.ok
@@ -9,6 +9,7 @@
 Testing creating Cipher Reject
 Testing creating CM U
 Testing creating SAPI Reject
+Testing creating Assignment Request
 Testing creating Assignment Complete
 Testing creating Assignment Complete (AoIP)
 Testing creating Assignment Failure