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>

 older verisons of glibc would build dl-sysdep as shared-only and dl-support as

 static-only.  alpha hooks in a cache variable via dl-auxv.h.  newer versions of

 glibc build dl-sysdep as both shared and static which means we now have symbol

 duplication for static builds with dl-sysdep and dl-support.  since dl-sysdep

 is both shared/static, there is no point in hooking dl-support anymore, so we

 can punt it.

 diff -durN glibc-2.13.orig/glibc-ports-2.13/sysdeps/unix/sysv/linux/alpha/dl-support.c glibc-2.13/glibc-ports-2.13/sysdeps/unix/sysv/linux/alpha/dl-support.c

 --- glibc-2.13.orig/glibc-ports-2.13/sysdeps/unix/sysv/linux/alpha/dl-support.c	2009-05-16 10:36:20.000000000 +0200

 +++ glibc-2.13/glibc-ports-2.13/sysdeps/unix/sysv/linux/alpha/dl-support.c	2009-11-13 00:50:53.000000000 +0100

 @@ -1,2 +1,1 @@

 -#include "dl-auxv.h"

  #include <elf/dl-support.c>