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 -dur binutils-2.17.old/bfd/opncls.c binutils-2.17/bfd/opncls.c

 --- binutils-2.17.old/bfd/opncls.c	2006-03-16 13:20:16.000000000 +0100

 +++ binutils-2.17/bfd/opncls.c	2007-05-01 18:26:11.000000000 +0200

 @@ -158,6 +158,13 @@

  {

    bfd *nbfd;

    const bfd_target *target_vec;

 +  struct stat s;

 +

 +  if (stat (filename, &s) == 0)

 +    if (S_ISDIR(s.st_mode)) {

 +      bfd_set_error (bfd_error_file_not_recognized);

 +      return NULL;

 +    }

    nbfd = _bfd_new_bfd ();

    if (nbfd == NULL)