Make read-only globals const, additional fixes

- Global variables that are never written to are marked const
- Remove some EXTERN declaration
- Remove unused `log_mem_multi` from logging module

executables/lte-softmodem.c

@@ -133,7 +133,6 @@ double sample_rate=30.72e6;
 double bw = 10.0e6;
 
 uint8_t dci_Format = 0;
-uint8_t agregation_Level =0xFF;
 
 uint8_t nb_antenna_tx = 1;
 uint8_t nb_antenna_rx = 1;