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 -durN glibc-2.7.orig/sysdeps/unix/sysv/linux/sh/sys/procfs.h glibc-2.7/sysdeps/unix/sysv/linux/sh/sys/procfs.h

 --- glibc-2.7.orig/sysdeps/unix/sysv/linux/sh/sys/procfs.h	2008-10-22 21:23:32.000000000 +0200

 +++ glibc-2.7/sysdeps/unix/sysv/linux/sh/sys/procfs.h	2008-10-22 21:41:54.000000000 +0200

 @@ -29,6 +29,7 @@

  #include <sys/types.h>

  #include <sys/ucontext.h>

  #include <sys/user.h>

 +#include <asm/ptrace.h>

  __BEGIN_DECLS

 diff -durN glibc-2.7.orig/sysdeps/unix/sysv/linux/sh/sys/user.h glibc-2.7/sysdeps/unix/sysv/linux/sh/sys/user.h

 --- glibc-2.7.orig/sysdeps/unix/sysv/linux/sh/sys/user.h	2008-10-22 21:23:32.000000000 +0200

 +++ glibc-2.7/sysdeps/unix/sysv/linux/sh/sys/user.h	2008-10-22 21:33:34.000000000 +0200

 @@ -21,8 +21,6 @@

  #include <features.h>

 -#include <asm/user.h>

 -

  #undef start_thread

  #endif  /* sys/user.h */