package/intel-microcode: install into images for early loading support

Microcode based security mitigation (E.G.  MDS) requires that the microcode
gets loaded very early. This can be handled by one of:

- Concatenating (a subset of) the intel-microcode files and write to
  kernel/x86/microcode/GenuineIntel.bin in the initrd.  Requires that the
  (first) initrd is external from the kernel and NOT compressed.

- Build (a subset of) the intel-microcode files into the kernel using the
  CONFIG_EXTRA_FIRMWARE option.

Install the microcode files into images to support these use cases (E.G.
through a post-build script for the initrd, or by pointing
CONFIG_EXTRA_FIRMWARE_DIR to ${BR_BINARIES_DIR}, similar to how we include
the .cpio image inside the kernel).

Notice that there may be licensing concerns when embedded non-GPL firmware
in the kernel.

Signed-off-by: Peter Korsgaard <peter@korsgaard.com>