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>

 Index: binutils-2.19.51.0.5/ld/ld.texinfo

 ===================================================================

 diff -durN binutils-2.20.1.orig/ld/ld.texinfo binutils-2.20.1/ld/ld.texinfo

 --- binutils-2.20.1.orig/ld/ld.texinfo	2009-07-06 15:48:51.000000000 +0200

 +++ binutils-2.20.1/ld/ld.texinfo	2010-08-17 19:32:16.000000000 +0200

 @@ -2026,8 +2026,9 @@

  systems may not understand them. If you specify

  @option{--enable-new-dtags}, the dynamic tags will be created as needed.

  If you specify @option{--disable-new-dtags}, no new dynamic tags will be

 -created. By default, the new dynamic tags are not created. Note that

 -those options are only available for ELF systems.

 +created. On Gentoo, by default, the new dynamic tags are created (this

 +differs from upstream behaviour). Note that those options are only

 +available for ELF systems.

  @kindex --hash-size=@var{number}

  @item --hash-size=@var{number}