Check crypto footer before offering to decrypt

Verify that we have a valid footer with proper magic before
setting things up for decryption to help prevent user confusion
when dealing with data partitions that fail to mount. Also check
to make sure that the block device for /data is present.

Change-Id: Ie87818fe4505a8bf71df7d3934c114e7328ef3ca
diff --git a/crypto/ics/cryptfs.h b/crypto/ics/cryptfs.h
index eee43b3..8c80376 100644
--- a/crypto/ics/cryptfs.h
+++ b/crypto/ics/cryptfs.h
@@ -76,6 +76,7 @@
 #ifdef __cplusplus
 extern "C" {
 #endif
+  int cryptfs_check_footer(void);
   int cryptfs_check_passwd(const char *pw);
 #ifdef __cplusplus
 }