Clean up panicf and introduce system_exception_wait to do further target tasks and wait for a button when an unrecoverable error has occurred (panic, UIE, etc.). Returning from that function should reboot or don't return from it. Move UIE and __div0 for ARM to its own file.

git-svn-id: svn://svn.rockbox.org/rockbox/trunk@19716 a1c6a512-1295-4272-9138-f99709370657

firmware/target/arm/tcc780x/system-tcc780x.c

@@ -287,6 +287,11 @@ void system_reboot(void)
     while (1);
 }
 
+void system_exception_wait(void)
+{
+    while ((GPIOA & 0x10) == 0); /* check for power button */
+}
+
 int system_memory_guard(int newmode)
 {
     (void)newmode;