sys-firmware/intel-microcode: rewrite to use iucode_tool.

sys-firmware/intel-microcode: rewrite to use iucode_tool.

- Use the Intel upstream 'iucode_tool' to process all of the input
  microcode, from both the text-format microcode.dat file and other
  inputs, then generate a clean set of split outputs & optionally also
  initramfs output.
- Allows easy inclusion of any future split-ucode releases for single
  CPUs.
- No longer uses intel-microcode2ucode.c from $FILESDIR.
- Expert users can use the new MICROCODE_SIGNATURES variable to install
  only a subset of microcodes on their system, as requested by bug
  643786.
- Avoids accidently bloated split-ucode files per bug #644100.
- USE=monolithic is no longer supported, please see iucode_tool for any
  fallback.
- USE=initramfs now writes to /boot/intel-uc.img (8.3-safe naming).

Fixes: https://bugs.gentoo.org/643786
Fixes: https://bugs.gentoo.org/644100
Package-Manager: Portage-2.3.16, Repoman-2.3.6
Signed-off-by: Robin H. Johnson <robbat2@gentoo.org>