gsm: TS 44.021 modified V.110 frame encoding/decoding support

3GPP TS 44.021 specifies the format for modified V.110 frames as used
on the GSM air (radio) interface.  Implement encoders and decoders for
this modified V.110 format.

Related: OS#1572
Change-Id: I60a2f2690459359437df20cf4da9043fa7c3ad11
diff --git a/src/gsm/Makefile.am b/src/gsm/Makefile.am
index 79c5f04..ce0b1f4 100644
--- a/src/gsm/Makefile.am
+++ b/src/gsm/Makefile.am
@@ -33,7 +33,7 @@
 			gsup.c gsup_sms.c gprs_gea.c gsm0503_conv.c oap.c gsm0808_utils.c \
 			gsm23003.c gsm23236.c mncc.c bts_features.c oap_client.c \
 			gsm29118.c gsm48_rest_octets.c cbsp.c gsm48049.c \
-			gad.c bsslap.c bssmap_le.c kdf.c iuup.c
+			gad.c bsslap.c bssmap_le.c kdf.c iuup.c gsm44021.c
 
 libgsmint_la_LDFLAGS = -no-undefined
 libgsmint_la_LIBADD = $(top_builddir)/src/core/libosmocore.la $(top_builddir)/src/isdn/libosmoisdn.la