Read out comport on x86 from BDA area

With this patch the comport are tried to read out from the BIOS Data Area
on x86 machines. The BDA Area are part of the first physical page and may
contain the non-default io ports of the comport.

When used together with 'bender' from the 'morbo' toolchain, even serial PCI
cards (also Intel's AMT Serial Over Line adapter) are detected automatically.

l4/pkg/bootstrap/server/src/platform/x86_pc.cc

@@ -612,8 +612,15 @@ public:
             printf("PCI IO port = %x\n", comport);
           }
 
-        if (comport == -1)
-          comport = 1;
+        if (comport == -1) {
+          /* try to read Bios Data Area (BDA) to get comport information */
+          unsigned short comport_count = (*((unsigned short *)0x410) >> 9) & 0x7;
+          if (comport_count)
+            comport = *((unsigned short *)0x400);
+          else
+            /* give up and try default values */
+            comport = 1;
+		}
 
         if (init_uart(comport, comirq, &du))
           printf("UART init failed\n");