conv: add convolutional coder for AMR SID UPDATE frames

AMR SID update frames are protected using an 1/4 convolutional coder,
wich is similar to the one used with 6,7 kbit voice frames. Except that
there is no puncturing and the length is different.

Change-Id: Ia35ed4178a7f0d816052b7e5d6478b93a1d9744f
Related: OS#2978
diff --git a/tests/conv/conv_gsm0503_test.ok b/tests/conv/conv_gsm0503_test.ok
index 39480ca..764bd43 100644
--- a/tests/conv/conv_gsm0503_test.ok
+++ b/tests/conv/conv_gsm0503_test.ok
@@ -190,6 +190,14 @@
 [..] Encoding / Decoding cycle : OK
 [..] Encoding / Decoding cycle : OK
 
+[+] Testing: gsm0503_tch_axs_sid_update
+[.] Input length  : ret =  49  exp =  49 -> OK
+[.] Output length : ret = 212  exp = 212 -> OK
+[.] Random vector checks:
+[..] Encoding / Decoding cycle : OK
+[..] Encoding / Decoding cycle : OK
+[..] Encoding / Decoding cycle : OK
+
 [+] Testing: gsm0503_mcs1_dl_hdr
 [.] Input length  : ret =  36  exp =  36 -> OK
 [.] Output length : ret = 108  exp = 108 -> OK