arm64: allow installing compressed image by default

arm64: allow installing compressed image by default

On arm64 we build compressed images, but "make install" by default will
install the old non-compressed one.  To actually get the compressed
image install, you need to use "make zinstall", which is not the usual
way to install a kernel.

Which may not sound like much of an issue, but when you deal with
multiple architectures (and years of your fingers knowing the regular
"make install" incantation), this inconsistency is pretty annoying.

But as Will Deacon says:
 "Sadly, bootloaders being as top quality as you might expect, I don't
  think we're in a position to rely on decompressor support across the
  board. Our Image.gz is literally just that -- we don't have a built-in
  decompressor (nor do I think we want to rush into that again after the
  fun we had on arm32) and the recent EFI zboot support solves that
  problem for platforms using EFI.

  Changing the default 'install' target terrifies me. There are bound to
  be folks with embedded boards who've scripted this and we could really
  ruin their day if we quietly give them a compressed kernel that their
  bootloader doesn't know how to handle :/"

So make this conditional on a new "COMPRESSED_INSTALL" option.

Cc: Catalin Marinas <catalin.marinas@arm.com>
Acked-by: Will Deacon <will@kernel.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>