Init serial port before using it for debug - also reinit after option rom.

Apparently, some VGA option roms will enable serial irqs - this could
    cause problems with spurious irqs from debug messages.
Also, improve debugging of option roms that fail the checksum check.
diff --git a/src/boot.c b/src/boot.c
index 09d28e8..3ee957b 100644
--- a/src/boot.c
+++ b/src/boot.c
@@ -200,6 +200,7 @@
 void VISIBLE16
 handle_18()
 {
+    debug_serial_setup();
     debug_enter(NULL, DEBUG_HDL_18);
     u16 seq = GET_EBDA(ipl.sequence) + 1;
     do_boot(seq);
@@ -209,6 +210,7 @@
 void VISIBLE16
 handle_19()
 {
+    debug_serial_setup();
     debug_enter(NULL, DEBUG_HDL_19);
     do_boot(0);
 }