Make disk access work.

Several bug fixes.
Move ata_detect from disk.c to ata.c.
diff --git a/src/boot.c b/src/boot.c
index 867b2fe..8595b7f 100644
--- a/src/boot.c
+++ b/src/boot.c
@@ -9,7 +9,7 @@
 #include "biosvar.h" // struct bregs
 #include "config.h" // CONFIG_*
 #include "cmos.h" // inb_cmos
-#include "disk.h" // ata_detect
+#include "ata.h" // ata_detect
 
 //--------------------------------------------------------------------------
 // print_boot_device