Replace merge.sh with a python script

This sits on top of !22 (closed)

Main patch here is the replacement of the merge.sh script with a python version that is, imo, a lot easier to read and understand and harder to get wrong. The handling of the HDR bits in the Makefile made me recoil in horror ;) I have rough plans to make the awk scripts etc also python scripts that can be debugged and tested easier.

Note that this introduces a dependency on python on the build - not sure how many of our build targets don't have python anyway...