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>

 This is the README for crosstool-NG

 Crosstool-NG follows the autoconf dance. So, to get you

 kick-started, just run:

     ./configure --help

 If you are using a development snapshot, you'll have to

 create the configure script, first. Just run:

     ./bootstrap

 You will find the documentation in the directory 'docs'.

 Here is a quick overview of what you'll find there:

   0 - Table of content

   1 - Introduction

   2 - Installing crosstool-NG

   3 - Configuring a toolchain

   4 - Building the toolchain

   5 - Using the toolchain

   6 - Toolchain types

   7 - Contributing

   8 - Internals

   A - Credits

   B - Known issues

   C - Misc. tutorials

 You can also point your browser at:

   http://crosstool-ng.org/

 Aloha!