imgtool: Assert "measurement value" is last in boot_record.py

'Measurement Value' added the 'properties' list initially,
rather than adding it separately later.
Added an assertion to ensure 'Measurement Value' remains as
the last item in the 'properties' list

Signed-off-by: Rustam Ismayilov <rustam.ismayilov@arm.com>
Change-Id: I106059c6c903c3d560477d5114d866f48590ad7e