Cleanup and refactor initial gNB configuration
This is follow-up on !3048 (merged). Major changes in this MR:
- Add config functions to reduce redundant code repeated in multiple places in the file
- Add macros to reduce redundant code in gNB configuration
- Refactor RCconfig_NR_NG
- Cleanup obsolete and commented out code
- Refactor includes in executables/nr-gnb.c
- Introduce platform type plmn_id_t for consistency across the stack: F1AP, NGAP can now use the same struct type
- And other minor formatting