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>

 --- glibc-2.13/sysdeps/unix/sysv/linux/kernel-features.h	2010-07-27 14:34:39.000000000 +0300

 +++ glibc-2.13/sysdeps/unix/sysv/linux/kernel-features.h.new	2011-03-10 18:54:37.686795979 +0200

 @@ -437,7 +437,8 @@

     the code.  On PPC they were introduced in 2.6.17-rc1,

     on SH in 2.6.19-rc1.  */

  #if __LINUX_KERNEL_VERSION >= 0x020611 \

 -    && (!defined __sh__ || __LINUX_KERNEL_VERSION >= 0x020613)

 +    && (!defined __sh__ || __LINUX_KERNEL_VERSION >= 0x020613) \

 +    && (!defined __alpha__)

  # define __ASSUME_ATFCTS	1

  #endif