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 gcc-4.2.1.orig/libstdc++-v3/include/ext/rope gcc-4.2.1/libstdc++-v3/include/ext/rope

 --- gcc-4.2.1.orig/libstdc++-v3/include/ext/rope	2006-10-17 13:56:21.000000000 +0200

 +++ gcc-4.2.1/libstdc++-v3/include/ext/rope	2007-08-03 20:34:23.000000000 +0200

 @@ -58,6 +58,9 @@

  #include <bits/allocator.h>

  #include <ext/hash_fun.h>

 +/* cope w/ index defined as macro, SuSv3 proposal */

 +#undef index

 +

  # ifdef __GC

  #   define __GC_CONST const

  # else

 diff -durN gcc-4.2.1.orig/libstdc++-v3/include/ext/ropeimpl.h gcc-4.2.1/libstdc++-v3/include/ext/ropeimpl.h

 --- gcc-4.2.1.orig/libstdc++-v3/include/ext/ropeimpl.h	2006-10-17 13:56:21.000000000 +0200

 +++ gcc-4.2.1/libstdc++-v3/include/ext/ropeimpl.h	2007-08-03 20:34:23.000000000 +0200

 @@ -54,6 +54,9 @@

  #include <ext/memory> // For uninitialized_copy_n

  #include <ext/numeric> // For power

 +/* cope w/ index defined as macro, SuSv3 proposal */

 +#undef index

 +

  _GLIBCXX_BEGIN_NAMESPACE(__gnu_cxx)

    using std::size_t;