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.

CHANGELOG.md

@@ -47,6 +47,12 @@
   - The web UI and the configuration upload now refuse a line the replay cannot take,
     instead of writing it to flash.
 
+- Startup
+  - Initialised and zero-initialised globals above xdata 0x4000 now get their values at boot.
+    Hardware reset leaves SFR 0x97 at 1, which maps xdata 0x4000-0xbfff to the flash image,
+    and it was cleared only in `main()`, after the C startup code had copied and cleared them.
+    This showed as a random web session timeout and LLDP blocked on some ports without config.
+
 ## Breaking changes
 
 - Config

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;

rtl837x_sfr.h

@@ -55,7 +55,7 @@ __sfr __at(0xbb) SFR_BANK_RET;
 
 __sfr __at(0x8e) CKCON;
 
-__sfr __at(0x97) SFR_97;	// HADDR?
+__sfr __at(0x97) MAP_FLASH_PAGE_TO_XDATA;	// 1 (hardware reset default): xdata 0x4000-0xbfff reads the flash image; 0: RAM
 __sfr __at(0xb9) SFR_b9;
 __sfr __at(0xba) SFR_ba;
 

rtlplayground.c

@@ -1567,6 +1567,15 @@ void check_and_flash_update_image(void)
 }
 
 
+/* Called by the SDCC startup code before it copies XINIT to XISEG and clears XSEG.
+ * Hardware reset starts us with MAP_FLASH_PAGE_TO_XDATA = 1, which maps xdata
+ * 0x4000-0xbfff to the flash image (reads return flash, writes are lost), so it
+ * must be 0 by then. */
+unsigned char __sdcc_external_startup(void)
+{
+	MAP_FLASH_PAGE_TO_XDATA = 0;
+	return 0;
+}
 
 void main(void)
 {
@@ -1577,7 +1586,6 @@ void main(void)
 	sbuf_ptr = 0;
 
 	CKCON = 0;	// Initial Clock configuration
-	SFR_97 = 0;	// HADDR?
 
 	// Set in managed mode:
 	SFR_b9 = 0x00;