Add option CONFIG_DISABLE_A20 to control disabling A20 on boot.

Almost nothing really wants A20 off today - so add a config option
    (and default it off) to control turning A20 off during 16bit
diff --git a/src/config.h b/src/config.h
index 70a9051..ccbfe08 100644
--- a/src/config.h
+++ b/src/config.h
@@ -49,6 +49,8 @@
 // Support calling int155f on each keyboard event
 #define CONFIG_KBD_CALL_INT15_4F 1
+// Disable A20 on 16bit boot
+#define CONFIG_DISABLE_A20 0
 // Support for int15c2 mouse calls
 #define CONFIG_PS2_MOUSE 1
 // If the target machine has multiple independent root buses, the
diff --git a/src/romlayout.S b/src/romlayout.S
index 73b7774..80b7a4b 100644
--- a/src/romlayout.S
+++ b/src/romlayout.S
@@ -188,10 +188,12 @@
         movw %ax, %fs
         movw %ax, %gs
         // disable a20
         inb $PORT_A20, %al
         andb $~A20_ENABLE_BIT, %al
         outb %al, $PORT_A20
         // Jump to 16bit mode
         ljmpw $SEG32_MODE16_CS, $1f