1. It's no need to modify LTE-RRC's ASN.1 file for enabling option group
   feature in generate_asn1.
2. It's no need to patch the C header files generated from
   LTE-RRC/S1AP/X2AP's ASN.1 in fix_asn1.
3. It's no need to pre-process S1AP/X2AP's ASN.1 for enabling information
   object class feature by asn1tostruct.py.

This commit requires an un-official version of asn1c which are
collaboration of asn1c community members and `build_helper` is modified
in this commit accordingly :

  https://github.com/brchiu/asn1c/tree/velichkov_s1ap_plus_option_group

This asn1c can also convert ASN.1 of 5G NR RRC (38.331, v15.1.0) and
generate C files with -findirect-choice command line option without
tweaking ASN.1 file.

Examples for how to use this asn1c for S1AP are in S1AP files, e.g.
s1ap_eNB_nas_procedures.c and s1ap_eNB_handler.c.

There is an modified openair-cn (based on tag 0.5.0) with this new asn1c at :

  https://github.com/brchiu/openair-cn/tree/new-asn1c

Basic S1AP interaction and data transfer operation between OAI eNB and
this modified openair-cn has been tested.

Note: Community version of asn1c is keeping evolved to provide better usage.