kbuild: buildtar: add comments about inconsistent package generation

kbuild: buildtar: add comments about inconsistent package generation

scripts/package/buildtar checks some kernel packages, and copies the
first image found. This may potentially produce an inconsistent (and
possibly wrong) package.

For instance, the for-loop for arm64 checks Image.{bz2,gz,lz4,lzma,lzo},
and vmlinuz.efi, then copies the first image found, which might be a
stale image created in a previous build.

When CONFIG_EFI_ZBOOT is enabled in the pristine source tree,
'make ARCH=arm64 tar-pkg' will build and copy vmlinuz.efi. This is the
expected behavior.

If you build the kernel with CONFIG_EFI_ZBOOT disabled, Image.gz will
be created, which will remain in the tree until you run 'make clean'.
Even if CONFIG_EFI_ZBOOT is turned on later, 'make ARCH=arm64 tar-pkg'
will copy stale Image.gz instead of the latest vmlinuz.efi, as Image.gz
takes precedence over vmlinuz.efi.

In summary, the code "[ -f ... ] && cp" does not consistently produce
the desired outcome.

Other packaging targets are deterministic; deb-pkg and rpm-pkg copies
${KBUILD_IMAGE}, which is determined by CONFIG options.

I removed [ -f ... ] checks from x86, alpha, parisc, and the default
because they have a single kernel image to copy. If it is missing, it
should be an error.

I did not modify the code for mips, arm64, riscv. Instead, I left some
comments. Eventually, someone may fix the code, or at the very least,
it may discourage the copy-pasting of incorrect code to another
architecture.

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Reviewed-by: Emil Renner Berthing <emil.renner.berthing@canonical.com>
Reviewed-by: Nicolas Schier <nicolas@fjasle.eu>