Add basic SI2quater support

* support for sending arbitrary static SI2quater.
* vty interface for neightbor EARFCNs specific to SI2quater.
* dynamic generation of SI2quater messages.
* unit test for SI2quater messages.

Fixes: OS#1630
diff --git a/openbsc/tests/gsm0408/gsm0408_test.ok b/openbsc/tests/gsm0408/gsm0408_test.ok
index 058563a..59319bf 100644
--- a/openbsc/tests/gsm0408/gsm0408_test.ok
+++ b/openbsc/tests/gsm0408/gsm0408_test.ok
@@ -62,4 +62,12 @@
 Allocated reference: 255
 Allocated reference: 0
 Allocated reference: 1
+Testing SYSINFO_TYPE_2quater generation:
+added EARFCN 1917 - generated SI2quater: [23] 59 06 07 c0 00 04 86 59 83 be c8 50 0b 2b 2b 2b 2b 2b 2b 2b 2b 2b 2b 
+added EARFCN 1932 - generated SI2quater: [23] 59 06 07 c0 00 04 86 59 83 be cc 1e 30 14 03 2b 2b 2b 2b 2b 2b 2b 2b 
+added EARFCN 1937 - generated SI2quater: [23] 59 06 07 c0 00 04 86 59 83 be cc 1e 31 07 91 a0 a0 2b 2b 2b 2b 2b 2b 
+added EARFCN 1945 - generated SI2quater: [23] 59 06 07 c0 00 04 86 59 83 be cc 1e 31 07 91 a8 3c c8 28 0b 2b 2b 2b 
+added EARFCN 1965 - generated SI2quater: [23] 59 06 07 c0 00 04 86 59 83 be cc 1e 31 07 91 a8 3c ca 0f 5a 0a 03 2b 
+added EARFCN 1967 - failed to generate SI2quater: Cannot allocate memory
+added EARFCN 1982 - failed to generate SI2quater: Cannot allocate memory
 Done.