Bug#1009807: Kernels with no compression support no longer work

[Raul Tambre <raul@tambre.ee>]

Package: initramfs-tools
Version: 0.141
Severity: important

Dear maintainer,

I was using a custom downstream kernel required for my hardware. The 
kernel had only CONFIG_RD_XZ enabled, which I wasn't aware of.
With version 0.141 initramfs creation fails with:

  Processing triggers for initramfs-tools (0.141) ...
  update-initramfs: Generating /boot/initrd.img-4.14.102-rt53
  grep: /boot/config-4.14.102-rt53: No such file or directory
  W: zstd compression (CONFIG_RD_ZSTD) not supported by kernel, using gzip
  grep: /boot/config-4.14.102-rt53: No such file or directory
  E: gzip compression (CONFIG_RD_GZIP) not supported by kernel
  update-initramfs: failed for /boot/initrd.img-4.14.102-rt53 with 1.
  dpkg: error processing package initramfs-tools (--configure):
   installed initramfs-tools package post-installation script 
subprocess returned error exit status 1

After commit 035190cc4385c0441dddc1bbaba000cf7f1f179b the code tries to 
default to gzip if the first method didn't work, and errors if so. 
Previously it would fall back to no compression.

While the setup is a bit exotic, falling back to no compression as a 
last resort (with a warning) would be what I expect.
For robustness it would probably even be ideal to test support for all 
that known methods in some order of preference and only then fall back 
to no compression.

All the best,
Raul