kernel/linux: use a custom kernel source directory

Allows using either a tarball or a directory as the custom kernel
source location.

Signed-off-by: Vincent BENOIT <sinseman44@gmail.com>
[yann.morin.1998@anciens.enib.fr: fix space damage, detailed commit message]
Signed-off-by: "Yann E. MORIN" <yann.morin.1998@anciens.enib.fr>

 #!/bin/sh

 tail -n+5 "${0}" |bzcat

 exit 0