kernel/linux: deprecate copying headers from pre-instaleld headers

No-one ever inquired about this feature. Due to the way it works, it is
rather unlikely that every user used a properly-formatted tarball, or a
properly structured directory as imput.

Beside, it is not really easy to maintain: I just spent one full day
remembering how it worked... Sigh... :-(

Just deprecate this, until next release, or until someone complains.

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

 mainmenu "The crosstool-NG configuration menu"

 source "config/configure.in"

 source "config/backend.in"

 source "config/global.in"

 source "config/target.in"

 source "config/toolchain.in"

 source "config/kernel.in"

 source "config/binutils.in"

 source "config/cc.in"

 source "config/libc.in"

 source "config/debug.in"

 source "config/companion_libs.in"

 source "config/companion_tools.in"

 source "config/test_suite.in"