Add code generator for convolutional codes

Add python utility to generate .c code with convolutional
encoder/decoder based on polynomial description of the code. If argument
given it'll be interpreted as intended output directory, otherwise
current working directory is used.

Codes for *CCH, CS2/3 and TCH/AFS are generated. Corresponding manual
implementations are removed from tests. This introduce build-time
dependency on python.

The main work for this patch was generously contributed by Sylvain
Munaut.

Fixes: OS#1629
diff --git a/.gitignore b/.gitignore
index f73a533..e3dbcac 100644
--- a/.gitignore
+++ b/.gitignore
@@ -100,6 +100,7 @@
 doc/html.tar
 
 src/crc*gen.c
+src/gsm/conv*gen.c
 include/osmocom/core/crc*gen.h
 include/osmocom/core/bit*gen.h