Do not include kernel.h in system.h.

system.h doesn't need it on its own and this change makes it less
dependant on Rockbox internals.

Change-Id: I4e1e4108a52a7b599627a829204eb82b392fc6d6

firmware/target/arm/as3525/debug-as3525.c

@@ -20,10 +20,11 @@
  ****************************************************************************/
 
 #include <stdbool.h>
+#include "system.h"
+#include "kernel.h"
 #include "button.h"
 #include "lcd.h"
 #include "font.h"
-#include "system.h"
 #include "cpu.h"
 #include "pl180.h"
 #include "ascodec.h"

firmware/target/arm/as3525/lcd-fuze.c

@@ -19,11 +19,12 @@
  * KIND, either express or implied.
  *
  ****************************************************************************/
-#include "config.h"
 
+#include "config.h"
 #include "cpu.h"
-#include "lcd.h"
 #include "system.h"
+#include "kernel.h"
+#include "lcd.h"
 #include "file.h"
 #include "clock-target.h"
 #include "dbop-as3525.h"

firmware/target/arm/as3525/sansa-fuzev2/button-fuzev2.c

@@ -21,6 +21,7 @@
 
 #include "config.h"
 #include "system.h"
+#include "kernel.h"
 #include "button.h"
 #include "backlight.h"
 

firmware/target/arm/pp/system-target.h

@@ -22,9 +22,10 @@
 #ifndef SYSTEM_TARGET_H
 #define SYSTEM_TARGET_H
 
+#include <stdbool.h>
 #include "config.h"
 #include "system-arm.h"
-#include <stdbool.h>
+#include "kernel.h"
 
 /* TODO: This header could be split in 2 */