Replace copy-firmware.sh; install files and generate metainfo.xml at build time

Currently we generate the metainfo.xml files and dh_install file lists from gencontrol.py, which requires installing files as part of source package preparation. This was mostly done because it was convenient to extend gencontrol.py, but it is not really necessary and it's annoying to have to run the file installation multiple times when building source and then binary packages.

The upstream copy-firmware.sh script, used to install firmware both at build time and when running gencontrol.py, doesn't work that well for us. I've had to patch it to cope with the fact that we exclude some files, and it's also rather slow.

This solves both problems by moving the firmware installation and metainfo.xml generation into a new install-files script which is only run at build time.