Specify use of non-free-firmware and contrib

• builder: Split 'free' into 2 properties
  • include_non_free_firmware
  • include_contrib
  • Closes: #209 (closed).
• vagrant: Include contrib for virtualbox-guest-utils
  • Previously, the contrib component would need to be specifically added using --add-release-component, or else virtualbox-guest-utils would not be installed in the image.
• amd64, i386: Add {amd64,intel}-microcode packages
  • CPU microcode updates are recommended for system stability, see https://wiki.debian.org/Microcode.
  • They are only needed on physical hardware, not VMs.

Tests:

• Build a raspberry3 image. raspi-firwmare package is downloaded and copied to firmware partition.
• Build a raspberry64 image. firmware-brcm80211 and raspi-firmware packages are installed.
• Build a vagrant image, and confirm that virtualbox-guest-utils is installed.
• Build an amd64 image, and see that it has amd64-microcode and intel-microcode packages installed.
• Build an i386 image, and see that it has amd64-microcode and intel-microcode packages installed.

Signed-off-by: James Valleroy jvalleroy@mailbox.org