Boot: Add TLV iterator API

Add TLV iterator API to MCUBoot as part of a partial synchronization
with the mainstream MCUBoot repository. The hash of the source commit:
61fd888a7f4d741714553f36839dd49fb0065731.

Change-Id: I817b199f4923433010253c4a201ada250f743aa8
Signed-off-by: David Vincze <david.vincze@arm.com>
diff --git a/bl2/ext/mcuboot/flash_map_legacy.c b/bl2/ext/mcuboot/flash_map_legacy.c
index 34b6b54..bbf800e 100644
--- a/bl2/ext/mcuboot/flash_map_legacy.c
+++ b/bl2/ext/mcuboot/flash_map_legacy.c
@@ -20,7 +20,7 @@
 /*
  * Original code taken from mcuboot project at:
  * https://github.com/JuulLabs-OSS/mcuboot
- * Git SHA of the original version: 4f0ea747c314547daa6b6299ccbd77ae4dee6758
+ * Git SHA of the original version: 61fd888a7f4d741714553f36839dd49fb0065731
  * Modifications are Copyright (c) 2019 Arm Limited.
  */