scsi: Add function and env var to report number of scsi drives

Add a new function to find out the number of available SCSI disks. Also
set the 'scsidevs' environment variable after each scan.

Signed-off-by: Stefan Reinauer <reinauer@chromium.org>
Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/include/scsi.h b/include/scsi.h
index 89ae45f..9681d19 100644
--- a/include/scsi.h
+++ b/include/scsi.h
@@ -189,6 +189,8 @@
 void scsi_init(void);
 void scsi_scan(int mode);
 
+/** @return the number of scsi disks */
+int scsi_get_disk_count(void);
 
 #define SCSI_IDENTIFY					0xC0  /* not used */