Zephyr 1.9 comes with an upgraded version of TinyCrypt. It is expected that other platforms will also upgrade at some point to this.
We have a ticket to move away from tinycrypt, and toward the ecc library it was based on. However, we need a solution for the 1.0 release, with a few possibilities:
Document that Zephyr 1.9 is not supported with ECDSA signatures
Embed an older version of TinyCrypt into the MCUboot tree
Upgrade the API used
The last one is probably not a good idea, since we need to be able to compile against other platforms still using the older version of Tinycrypt. Incorporating a version of tinycrypt into our tree is probably the cleanest solution, and allows us to upgrade when we feel it is time.