Limit Xosera config to 2 UARTS; set $A0 base addr.

The consenus is that the best default address for Xosera
is $A0, so make that the default.

However, in that case we need to limit the number of UARTs
that are probed to two because the probe for a third UART
writes unlucky values to Xosera (at $A3) and causes it to
reconfigure itself and lock up the bus for a time.

Thanks to Wayne for his help in debugging this.

Source/HBIOS/Config/RCZ80_xosera.asm

@@ -41,14 +41,21 @@
 ; ".SET" TO OVERRIDE SETTINGS.  THE ASSEMBLER WILL ERROR IF YOU ATTEMPT
 ; TO USE ".EQU" BECAUSE IT WON'T LET YOU REDEFINE A SETTING WITH ".EQU".
 ;
-; THIS FILE ENABLES THE XOSERA DRIVER WITH A BASE ADDRESS Of 0XE0
-; AND DISPLAY SIZE OF 80 COLUMNS X 30 ROWS
+; THIS FILE ENABLES THE XOSERA DRIVER WITH A BASE ADDRESS Of $A0 AND 
+; DISPLAY SIZE OF 80 COLUMNS X 30 ROWS.
 ;
 #INCLUDE "Config/RCZ80_std.asm"		; INHERIT FROM OFFICIAL BUILD SETTINGS
 ;
 XOSENABLE	.SET	TRUE		; XOSERA: ENABLE XOSERA VIDEO DRIVERS (XOSERA.ASM)
-XOS_BASE	.SET	$20		; XOSERA: I/O BASE ADDRESS (REQUIRES 32 BYTES)
+XOS_BASE	.SET	$A0		; XOSERA: I/O BASE ADDRESS (REQUIRES 32 BYTES)
 XOSSIZ		.SET	V80X30		; XOSERA: DISPLAY FORMAT [V80X30|V80X60]
 ;
 AUTOCON		.SET	FALSE		; ENABLE CONSOLE TAKEOVER AT LOADER PROMPT
 VDAEMU_SERKBD	.SET	$0		; VDA EMULATION: SERIAL KBD UNIT #, OR $FF FOR HW KBD
+;
+; WHEN A XOSERA BOARD IN IS THE SYSTEM, LIMIT THE NUMBER OF UARTS THAT ARE PROBED
+; TO TWO, BECAUSE THE PROBE TO DETECT A THIRD UART WRITES UNLUCKY VALUES TO 
+; XOSERA THAT CAUSE IT TO RECONFIGURE ITSELF AND LOCK UP THE BUS FOR A TIME. IF
+; YOU NEED MORE THAN TWO UARTS, YOU WILL NEED TO MOVE XOSERA OUT OF THE $A0-$BF
+; I/O ADDRESS REGION.
+UARTCNT		.SET	2