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-2.12.1.orig/sysdeps/unix/sysv/linux/kernel-features.h glibc-2.12.1/sysdeps/unix/sysv/linux/kernel-features.h

 --- glibc-2.12.1.orig/sysdeps/unix/sysv/linux/kernel-features.h	2009-05-09 08:54:20.000000000 +0200

 +++ glibc-2.12.1/sysdeps/unix/sysv/linux/kernel-features.h	2009-11-13 00:50:45.000000000 +0100

 @@ -386,6 +386,11 @@

  # define __ASSUME_GETDENTS32_D_TYPE	1

  #endif

 +/* Starting with version 2.6.4, alpha stat64 syscalls are available.  */

 +#if __LINUX_KERNEL_VERSION >= 0x020604 && defined __alpha__

 +# define __ASSUME_STAT64_SYSCALL	1

 +#endif

 +

  /* Starting with version 2.5.3, the initial location returned by `brk'

     after exec is always rounded up to the next page.  */

  #if __LINUX_KERNEL_VERSION >= 132355