Allow to set custom DTB/OS_CALL addresses

Setting those from command line during compilation allows
to create a custom setup without the need of modifying the
sources.

src/main/c/emulator/src/config.h

@@ -3,7 +3,13 @@
 
 //#define QEMU
 #define SIM
+
+#ifndef OS_CALL
 #define OS_CALL 0xC0000000
+#endif
+
+#ifndef DTB
 #define DTB 0xC3000000
+#endif
 
 #endif