Provide a mechanism for key ID handling (MCUBOOT_BUILTIN_KEY)

[davidvincze]

As an initial solution the responsibility of handling key IDs was delegated to the underlying crypto library. This is far from ideal, MCUBoot forced an initial mapping between the image index and key ID (image_index +1). Handling of the key IDs should be independent of the bootloader and crypto code to be able to provide more generic solutions. Proposed solutions are:

1. add a new IMAGE_TLV_KEYID TLV type (this can be set as the EXPECTED_KEY_TLV),
2. provide a HAL API to do the mapping: ~bootutil_get_keyid_from_img_index()

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