kernel/linux: allow headers from full custom source tree

Accept a local tarball name as the source of the Linux kernel headers,
rather than forcing the user to use either an upstream tarball, or a
local pre-installed headers tree.

#!/bin/sh

# Needed for systems without gettext

$* -xc -o /dev/null - > /dev/null 2>&1 << EOF

#include <libintl.h>

int main()

{

	gettext("");

	return 0;

}

EOF

if [ ! "$?" -eq "0"  ]; then

	echo -DKBUILD_NO_NLS;

fi