startup: clear MAP_FLASH_PAGE_TO_XDATA before the C runtime initialises xdata

Hardware reset starts the firmware with SFR 0x97 = 1, which maps xdata
0x4000-0xbfff to the flash image: reads return flash bytes, writes are
lost. main() cleared it only after __mcs51_genXINIT and the XSEG clear
had run, so XISEG (initialised globals) and the part of XSEG above
0x4000 kept stale RAM contents, e.g. session_timeout = 4484 and LLDP
blocked on logical ports 0, 3 and 5. Clear it in
__sdcc_external_startup(), which SDCC calls before both.

Rename SFR_97 to MAP_FLASH_PAGE_TO_XDATA.

installer/installer.c

@@ -348,7 +348,7 @@ void installer(void)
 {
 
 	CKCON = 0;	// Initial Clock configuration
-	SFR_97 = 0;	// HADDR?
+	MAP_FLASH_PAGE_TO_XDATA = 0;	// HADDR?
 
 	// Set in managed mode:
 	SFR_b9 = 0x00;