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 ltrace-0.4.orig/configure ltrace-0.4/configure

 --- ltrace-0.4.orig/configure	2008-10-23 23:19:38.000000000 +0200

 +++ ltrace-0.4/configure	2008-10-23 23:19:06.000000000 +0200

 @@ -1392,6 +1392,9 @@

  HOST_OS="$host_os"

 +case "${HOST_OS}" in

 +  linux-*) HOST_OS=linux-gnu;;

 +esac

  ac_ext=c

 diff -durN ltrace-0.4.orig/configure.ac ltrace-0.4/configure.ac

 --- ltrace-0.4.orig/configure.ac	2008-10-23 23:19:43.000000000 +0200

 +++ ltrace-0.4/configure.ac	2008-10-23 23:18:44.000000000 +0200

 @@ -7,6 +7,9 @@

  dnl Check host system type

  AC_CANONICAL_HOST

  HOST_OS="$host_os"

 +case "${HOST_OS}" in

 +  linux-*) HOST_OS=linux-gnu;;

 +esac

  AC_SUBST(HOST_OS)

  dnl Checks for programs.