Allow mcuboot tool to use cbor2 additional to cbor

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.

tools/mcuboot/imgtool/boot_record.py

@@ -14,6 +14,9 @@
 # limitations under the License.
 
 from enum import Enum
+try:
+    import cbor2 as cbor
+except:
     import cbor