micro-add: Refactor, rename 'first'->'comparison'
micro-add: Refactor, rename 'first'->'comparison'

Currently we choose the first (oldest) micro iamge as the appropriate
one to compare against when capturing. This may not always be the case,
rename everything to refer to the 'comparison' image instead of the
'first' image.

Test Plan:
$ ./meta/run_tests.sh
.. exercise micro-add manually ..

aevri/mel,aevri/mel