Add overlay file option to rgblink (-O)

This option takes a file and places fixed sections on top of it.
Should prove useful for patches and partial disassemblies.

include/link/mylink.h

@@ -10,6 +10,7 @@
 extern SLONG options;
 #define OPT_SMALL	0x01
 #define OPT_SMART_C_LINK	0x02
+#define OPT_OVERLAY	0x04
 
 enum eRpnData {
 	RPN_ADD = 0,

include/link/output.h

@@ -2,6 +2,7 @@
 #define	RGBDS_LINK_OUTPUT_H
 
 void out_Setname(char *tzOutputfile);
+void out_SetOverlayname(char *tzOverlayfile);
 void Output(void);
 
 #endif