Allow mcuboot tool to use cbor2 additional to cbor

[NeroBurner]

The mcuboot imgtool uses the python module cbor.

An equivalent and updated package cbor2 requires Python 3.7 or newer.

The arch packages provide a package python-cbor2, but no package for cbor.

This patch makes it possible to use the system package by adding support for the cbor2 package additionally to the cbor package.

[github-actions[bot]]

Build size and comparison to main:	Section	Size	Difference
text	407308B	0B
data	940B	0B
bss	63348B	0B