cpu: drop cpu_init() from public headers

Boards are no longer supposed fo call the function, so drop it from
public header files.

cpu/avr8_common/include/cpu.h

@@ -161,11 +161,6 @@ static inline int avr8_is_uart_tx_pending(void)
  */
 void avr8_exit_isr(void);
 
-/**
- * @brief Initialization of the CPU
- */
-void cpu_init(void);
-
 /**
  * @brief Initialization of the CPU clock
  */

cpu/avr8_common/startup.c

@@ -38,6 +38,7 @@
  * @brief functions for initializing the board, std-lib and kernel
  */
 extern void board_init(void);
+extern void cpu_init(void);
 extern void kernel_init(void);
 extern void __libc_init_array(void);
 

cpu/cortexm_common/include/cpu.h

@@ -66,11 +66,6 @@ extern "C" {
  */
 #define CORTEXM_SCB_CPACR_FPU_ACCESS_FULL         (0x00f00000)
 
-/**
- * @brief   Initialization of the CPU
- */
-void cpu_init(void);
-
 /**
  * @brief   Initialize Cortex-M specific core parts of the CPU
  *

cpu/cortexm_common/vectors_cortexm.c

@@ -182,6 +182,7 @@ void reset_handler_default(void)
 #endif
 
     /* initialize the CPU */
+    extern void cpu_init(void);
     cpu_init();
 
     /* initialize the board (which also initiates CPU initialization) */

cpu/mips32r2_common/cpu.c

@@ -68,6 +68,7 @@ void cpu_init(void)
     /* trigger static peripheral initialization */
     periph_init();
 }
+
 void mips_start(void)
 {
     cpu_init();

cpu/mips32r2_common/include/cpu.h

@@ -50,11 +50,6 @@ static inline void cpu_print_last_instruction(void)
     /* This function must exist else RIOT won't compile */
 }
 
-/**
- * @brief Initialize the CPU, set IRQ priorities
- */
-void cpu_init(void);
-
 /**
  * @brief   Trigger a conditional context scheduler run / context switch
  *

cpu/msp430_common/include/cpu.h

@@ -117,11 +117,6 @@ static inline void __attribute__((always_inline)) __exit_isr(void)
     __restore_context();
 }
 
-/**
- * @brief   Initialize the cpu
- */
-void msp430_cpu_init(void);
-
 /**
  * @brief   Print the last instruction's address
  *

cpu/msp430_common/startup.c

@@ -33,6 +33,7 @@
 #endif
 
 extern void board_init(void);
+extern void msp430_cpu_init(void);
 
 __attribute__((constructor)) static void startup(void)
 {

cpu/riscv_common/include/cpu.h

@@ -29,11 +29,6 @@
 extern "C" {
 #endif
 
-/**
- * @brief   Initialization of the CPU
- */
-void cpu_init(void);
-
 /**
  * @brief   Initialize rv32i specific core parts of the CPU
  *