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>

 Patch from upstream:

 http://strace.cvs.sourceforge.net/viewvc/strace/strace/linux/arm/syscallent.h?r1=1.17&r2=1.18&sortby=file

 2008-11-13  Kirill A. Shutemov  <kirill@shutemov.name>

     * linux/arm/syscallent.h: Fix build on ARM EABI which does not

     provide syscalls socketcall and ipc.

 --- strace-4.5.18.orig/linux/arm/syscallent.h	2008/12/29 20:04:15	1.17

 +++ strace-4.5.18/linux/arm/syscallent.h	2009/01/01 23:20:38	1.18

 @@ -431,6 +431,7 @@

  	{ 5,	0,	printargs,		"SYS_398"	}, /* 398 */

  	{ 5,	0,	printargs,		"SYS_399"	}, /* 399 */

 +#ifndef __ARM_EABI__

  #if SYS_socket_subcall != 400

   #error fix me

  #endif

 @@ -481,3 +482,4 @@

  	{ 4,	TI,	sys_shmdt,		"shmdt"		}, /* 440 */

  	{ 4,	TI,	sys_shmget,		"shmget"	}, /* 441 */

  	{ 4,	TI,	sys_shmctl,		"shmctl"	}, /* 442 */

 +#endif