build: Add support for decompressing bzLinux kernels
build: Add support for decompressing bzLinux kernels

Support plain gzip'ed files, as used on arm64, and bzImage with
embedded gzip'ed kernel, as used on x86.

Signed-off-by: Rolf Neugebauer <bc755ac45a0e799d1d490cba76febc56d80cc5fc@rneugeba.io>

justincormack/linuxkit,linuxkit/linuxkit,konstruktoid/linuxkit,djs55/linuxkit,konstruktoid/linuxkit,JohnnyLeone/linuxkit,deitch/linuxkit,deitch/linuxkit,djs55/linuxkit,yankcrime/linuxkit,djs55/linuxkit,deitch/linuxkit,rn/linuxkit,linuxkit/linuxkit,konstruktoid/linuxkit,linuxkit/linuxkit,deitch/linuxkit,deitch/linuxkit,konstruktoid/linuxkit,yankcrime/linuxkit,rn/linuxkit,djs55/linuxkit,rn/linuxkit,JohnnyLeone/linuxkit,justincormack/linuxkit,linuxkit/linuxkit,justincormack/linuxkit,JohnnyLeone/linuxkit,furious-luke/linuxkit,konstruktoid/linuxkit,yankcrime/linuxkit,yankcrime/linuxkit,furious-luke/linuxkit,yankcrime/linuxkit,rn/linuxkit,justincormack/linuxkit,JohnnyLeone/linuxkit,furious-luke/linuxkit,rn/linuxkit,djs55/linuxkit,furious-luke/linuxkit,linuxkit/linuxkit,konstruktoid/linuxkit,JohnnyLeone/linuxkit,yankcrime/linuxkit,justincormack/linuxkit,justincormack/linuxkit,furious-luke/linuxkit