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>

 2008-04-28  Hiroki Kaminaga  <kaminaga@sm.sony.co.jp>

 	[BZ 6740]

 	nptl/sysdeps/powerpc/tcb-offsets.sym (PRIVATE_FUTEX_OFFSET):

 	Guard sym definition with #ifndef __ASSUME_PRIVATE_FUTEX.

 --- a/nptl/sysdeps/powerpc/tcb-offsets.sym

 +++ b/nptl/sysdeps/powerpc/tcb-offsets.sym

 @@ -15,4 +15,6 @@

  PID				thread_offsetof (pid)

  TID				thread_offsetof (tid)

  POINTER_GUARD			(offsetof (tcbhead_t, pointer_guard) - TLS_TCB_OFFSET - sizeof (tcbhead_t))

 +#ifndef __ASSUME_PRIVATE_FUTEX

  PRIVATE_FUTEX_OFFSET		thread_offsetof (header.private_futex)

 +#endif