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>

 Dmitry V. Levin [Tue, 3 Nov 2009 16:49:49 +0000]

 Fix check for linux/netlink.h on Linux 2.6.32-rc5+

 * configure.ac (AC_CHECK_HEADERS): In check for linux/netlink.h, include 

   sys/socket.h instead of linux/socket.h beforehand.

 diff --git a/configure.ac b/configure.ac

 --- a/configure.ac

 +++ b/configure.ac

 @@ -258,7 +258,7 @@ AC_CHECK_HEADERS([ \

  ], [], [])

  AC_CHECK_HEADERS([linux/icmp.h linux/in6.h linux/netlink.h linux/if_packet.h],

                   [], [], [#include <stddef.h>

 -#include <linux/socket.h>])

 +#include <sys/socket.h>])

  AC_CHECK_HEADERS([asm/sigcontext.h], [], [], [#include <signal.h>])

  AC_CHECK_TYPES([struct sigcontext_struct,

  		struct sigcontext],,, [#include <signal.h>])