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>

diff -dur glibc-cvs-2.9.orig/debug/readlink_chk.c glibc-cvs-2.9/debug/readlink_chk.c

--- glibc-cvs-2.9.orig/debug/readlink_chk.c	2005-03-01 01:41:15.000000000 +0100

+++ glibc-cvs-2.9/debug/readlink_chk.c	2009-06-01 10:59:37.000000000 +0200

@@ -25,7 +25,7 @@

 ssize_t

-__readlink_chk (const char *path, void *buf, size_t len, size_t buflen)

+__readlink_chk (const char *path, char *buf, size_t len, size_t buflen)

 {

   if (len > buflen)

     __chk_fail ();

diff -dur glibc-cvs-2.9.orig/debug/readlinkat_chk.c glibc-cvs-2.9/debug/readlinkat_chk.c

--- glibc-cvs-2.9.orig/debug/readlinkat_chk.c	2006-04-24 18:56:12.000000000 +0200

+++ glibc-cvs-2.9/debug/readlinkat_chk.c	2009-06-01 11:07:26.000000000 +0200

@@ -21,7 +21,7 @@

 ssize_t

-__readlinkat_chk (int fd, const char *path, void *buf, size_t len,

+__readlinkat_chk (int fd, const char *path, char *buf, size_t len,

 		  size_t buflen)

 {

   if (len > buflen)