Move screen management code to screen.c and fix cache-related issues

- Screen brightness is now updated as soon as the user selects a brightness option, on all boot environments
- Payloads can now be 1KB bigger
- Some cache-related bugs may have been fixed

loader/source/main.c

@@ -4,7 +4,8 @@ void main(void)
 {
     void *payloadAddress = (void *)0x23F00000;
 
-    memcpy(payloadAddress, (void*)0x24F00000, *(u32 *)0x24FFFB04);
+    memcpy(payloadAddress, (void*)0x24F00000, *(u32 *)0x24FFFF04);
+    cleanInvalidateDCacheAndDMB(); //Ensure that all memory transfers have completed and that the data cache has been flushed 
 
     ((void (*)())payloadAddress)();
 }