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 -ru ltrace-0.5.3.org/configure ltrace-0.5.3/configure

 --- ltrace-0.5.3.org/configure	2011-08-21 18:55:15.000000000 +0200

 +++ ltrace-0.5.3/configure	2011-08-21 18:54:46.000000000 +0200

 @@ -15,8 +15,12 @@

  echo $PACKAGE_VERSION

  echo -n "checking HOST_OS... "

 -HOST_OS=$( uname -s )

 -if [ "$HOST_OS" = "Linux" ]

 +if [ -z "$HOST_OS" ] ; then

 +  HOST_OS=$( uname -s )

 +else

 +  echo -n "using preset: "

 +fi

 +if [ "$HOST_OS" = "Linux" -o "$HOST_OS" = "linux" ]

  then

    HOST_OS="linux-gnu"

  fi