Fix another broken symlink and avoid creating more of them

Bug #1118195 was caused by dedupe-firmware.sh symlinking a file that we included in a binary package to an identical file that we didn't yet include.

To avoid similar bugs in future:

• Switch to running dedupe-firmware.sh on each package installation directory, not on debian/build/install
• Add a check for broken symlinks, as they may also be defined explicitly in WHENCE

This did find one more broken symlink in intel-misc, so fix that by including the new firmware it refers to.