Use srec_cat (or a similar tool) instead of hexmate to merge bootloader and application images

[bessman]

We currently use hexmate, a microchip tool, to merge the bootloader and application HEX files. See

pslab-firmware/.github/workflows/main-builder.yml

Line 90 in 0285f6d

- name: Combine hex files

.

Hexmate is proprietary and is only distributed as part of either the xc8 compiler or the mplab-x IDE. It would be preferable to use a free, standalone tool instead.

There are many free tools available for manipulating eeprom images. SRecord is a collection of such tools, including srec_cat which should be suitable for this task. Another option is to use the bincopy python package. Many other alternatives likely exists.

The task is to duplicate the result of merging with hexmate. The output of whichever tool is chosen should be binary identical with hexmate's output. The outputs do not necessarily need to have identical text representations (hex files are plain text).