bios_emulator: Add an option to enable debugging

At present there are DEBUG options spread around the place. If you enable
one and not another you can end up with an emulator that does not work,
since each file can have a different view of what the registers look like.
To fix this, create a global CONFIG_X86EMU_DEBUG option that keeps
everything consistent.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/drivers/bios_emulator/include/x86emu/regs.h b/drivers/bios_emulator/include/x86emu/regs.h
index a7fedd2..2934129 100644
--- a/drivers/bios_emulator/include/x86emu/regs.h
+++ b/drivers/bios_emulator/include/x86emu/regs.h
@@ -282,7 +282,7 @@
 	u8 intno;
 	volatile int intr;	/* mask of pending interrupts */
 	int debug;
-#ifdef DEBUG
+#ifdef CONFIG_X86EMU_DEBUG
 	int check;
 	u16 saved_ip;
 	u16 saved_cs;