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/memory.c

@@ -7,4 +7,9 @@ void memcpy(void *dest, const void *src, u32 size)
 
     for(u32 i = 0; i < size; i++)
         destc[i] = srcc[i];
+}
+
+void cleanInvalidateDCacheAndDMB(void)
+{
+    ((void (*)())0xFFFF0830)(); //Why write our own code when it's well implemented in the unprotected bootROM?
 }