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

 # 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