cpu/cc2538: don't pollute global namespace with cc2538_rfcore.h

`cc2538_rfcore.h` contains enums with non-prefixed single-word constants.
Those cause ugly naming conflicts down the line.
Only include the file when needed, don't include it in `cpu_conf.h` which
gets pulled in everywhere.

cpu/cc2538/include/cc2538_rf.h

@@ -25,6 +25,7 @@
 #include <stdbool.h>
 
 #include "board.h"
+#include "cc2538_rfcore.h"
 
 #include "net/ieee802154.h"
 #include "kernel_defines.h"

cpu/cc2538/include/cpu_conf.h

@@ -27,7 +27,6 @@
 #include "cc2538_gptimer.h"
 #include "cc2538_soc_adc.h"
 #include "cc2538_ssi.h"
-#include "cc2538_rfcore.h"
 #include "cc2538_sys_ctrl.h"
 
 #ifdef __cplusplus

cpu/cc2538/periph/hwrng.c

@@ -25,6 +25,7 @@
 #include "vendor/hw_soc_adc.h"
 
 #include "cpu.h"
+#include "cc2538_rfcore.h"
 #include "periph/hwrng.h"
 
 #define ENABLE_DEBUG 0