Split building and installing intel-ucode.img to pacman hook

As an alternative to #3 (closed)

This does not change anything by default. It offers the possibility to mask the pacman hook and skip building and copying intel-ucode.img if desired.

It also splits installation of intel-ucode-with-caveats to /usr/share/ to cover #3 (comment 213794) so this is still included in intel-ucode.img, but not in /usr/lib/firmware/

Still happy to hear your thoughts even if this is not desired.