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>

 2008-06-07  Mike Frysinger  <vapier@gentoo.org>

 	* sysdeps/unix/sysv/linux/alpha/sys/user.h: Copy page.h definitions

 	from the kernel header to sys/user.h and remove the #include of

 	sys/user.h.

 --- a/sysdeps/unix/sysv/linux/alpha/sys/user.h

 +++ b/sysdeps/unix/sysv/linux/alpha/sys/user.h

 @@ -23,7 +23,6 @@

     only. Don't read too much into it. Don't use it for anything other

     than gdb/strace unless you know what you are doing. */

 -#include <asm/page.h>

  #include <asm/reg.h>

  struct user

 @@ -41,6 +40,9 @@ struct user

    char u_comm[32];				/* user command name */

  };

 +#define PAGE_SHIFT		13

 +#define PAGE_SIZE		(1 << PAGE_SHIFT)

 +#define PAGE_MASK		(~(PAGE_SIZE-1))

  #define NBPG			PAGE_SIZE

  #define UPAGES			1

  #define HOST_TEXT_START_ADDR	(u.start_code)