kernel/linux: simplify custom tarball handling

Simplify the way the custom tarball is handled:
- fake version="custom"
- at download, simply link the custom tarball to:
"linux-custom.${custom_extension}"
- at extract, the above allows to simply extract "linux-${LINUX_VERSION}"
where LINUX_VERISON is set to the fake version="custom"

Not that much convoluted, in fact... :-/

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@anciens.enib.fr>

 COPYING