imgtool: prefer cbor2 over cbor

The cbor module is unmaintained, with the last release in 2016[1]. The
cbor2 module however is under active development and was last released
just last month[2].

As the APIs are identical, we can import cbor2 and if that fails fall
back to cbor.

[1] https://pypi.org/project/cbor/#history
[2] https://pypi.org/project/cbor2/#history



Closes #1189

Signed-off-by: Ross Burton <ross.burton@arm.com>
Change-Id: Iaf2d0df625a200a5cebf72dec4a89877a26194ea