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>

 Original patch from gentoo: gentoo/src/patchsets/gcc/4.3.1/gentoo/11_all_gcc-netbsd-symbolic.patch

 http://bugs.gentoo.org/122698

 diff -durN gcc-4.3.1.orig/gcc/config/netbsd-elf.h gcc-4.3.1/gcc/config/netbsd-elf.h

 --- gcc-4.3.1.orig/gcc/config/netbsd-elf.h	2007-09-03 18:14:04.000000000 +0200

 +++ gcc-4.3.1/gcc/config/netbsd-elf.h	2008-06-10 14:57:48.000000000 +0200

 @@ -82,6 +82,7 @@

  #define NETBSD_LINK_SPEC_ELF \

    "%{assert*} %{R*} %{rpath*} \

     %{shared:-shared} \

 +   %{symbolic:-Bsymbolic} \

     %{!shared: \

       -dc -dp \

       %{!nostdlib: \