Looking for custom tlv documentation

[Olstyle]

I am trying to add additional data to my images, which should be used as an additional information about whether the image should be booted or not (one hardware with one bootloader becoming different devices later in the production process). So the data should be combined with the image and needs to be signed as part of the image.
Looking at the current documentation of imgtool, this looks just like what I need:

--custom-tlv [tag] [value]      Custom TLV that will be placed into
                                  protected area. Add "0x" prefix if the value
                                  should be interpreted as an integer,
                                  otherwise it will be interpreted as a
                                  string. Specify the option multiple times to
                                  add multiple TLVs.

But which tags are actually free to use?
Or don't they overlap with standard tlv entries?
And which interfaces should be used to access such a tlv?

@michalek-no maybe you can shed some light on this since it looks like you added the info about that option recently. So I guess you worked with it as well?

[taltenbach]

The list of the TLV types used by MCUboot can be found in image.h. In that header, there are areas defined for vendor-specific types:

mcuboot/boot/bootutil/include/bootutil/image.h

Lines 134 to 143 in 6cbea0a

	/*
	* vendor reserved TLVs at xxA0-xxFF,
	* where xx denotes the upper byte
	* range. Examples:
	* 0x00a0 - 0x00ff
	* 0x01a0 - 0x01ff
	* 0x02a0 - 0x02ff
	* ...
	* 0xffa0 - 0xfffe
	*/

The API for iterating over the TLVs of an image can be found at the end of the same header.

[Olstyle]

Thank you. Not so good for external use was the fact that the interfaces expect completely loaded images instead of index + slot.
I have now used the boot_perform_update_hook as an entry point for my additional check, since I can still interrupt the actual update there and the hook directly has all the pointers necessary for tlv iteration ( boot_image_check_hook doesn't).