Makefile, copy-firmware: support xz/zstd compressed firmware

Makefile, copy-firmware: support xz/zstd compressed firmware

The kernel has supported compressed firmware for quite some time. So
let's add a couple of targets to produce that. In practical terms this
means it we'll use ~5x times less space on disk.

Reportedly the amd ucode, needs to be uncompressed _within_ the
initrd in order to work. Using compressed ucode in late load just works.
Ideally this will be addressed by the initrd generators, but considering
the files are tiny in size let's skip the compression.

v2
 - commit message, skip compression for files annotated as Raw

v3
 - rebase

[Drop extra verbose statement in zstd case, Josh Boyer
<jwboyer@kernel.org>]

Cc: David Woodhouse <dwmw2@infradead.org>
Signed-off-by: Emil Velikov <emil.l.velikov@gmail.com>
Signed-off-by: Josh Boyer <jwboyer@kernel.org>