kernel/linux: fix using custom location

Currently, extract and patch are skipped as thus:
- using a custom directory of pre-installed headers
- a correctly named directory already exists

Otherwise, extract and patch are done.

The current second condition is wrong, because it allows the following
sequence to happen:
- a non-custom kernel is used
- a previous build only partially extracted the non-custom sources
- that p[revious build broke during extraction (eg. incomplete tarball...)
- a subsequent build will find a properly named directory, and will
thus skip extract and patch, which is wrong

Fix that by following the conditions in this table:

Type | Extract | Patch
----------------------+---------+-------
Pre-installed headers | N | N
custom directory | N | N
custom tarball | Y | N
mainstream tarball | Y | Y

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: David Holsgrove <david.holsgrove@xilinx.com>

 alpha does not have a __NR_creat

 http://bugs.gentoo.org/227275

 http://sourceware.org/bugzilla/show_bug.cgi?id=6650

 diff -durN glibc-2.12.1.orig/sysdeps/unix/sysv/linux/wordsize-64/creat64.c glibc-2.12.1/sysdeps/unix/sysv/linux/wordsize-64/creat64.c

 --- glibc-2.12.1.orig/sysdeps/unix/sysv/linux/wordsize-64/creat64.c	2007-11-10 20:34:26.000000000 +0100

 +++ glibc-2.12.1/sysdeps/unix/sysv/linux/wordsize-64/creat64.c	2009-11-13 00:50:51.000000000 +0100

 @@ -1 +1,5 @@

  /* Defined as alias for the syscall.  */

 +#include <sysdep.h>

 +#ifndef __NR_creat

 +#include "../../../../../io/creat64.c"

 +#endif