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 strace-4.5.16.orig/net.c strace-4.5.16/net.c

 --- strace-4.5.16.orig/net.c	2006-12-13 20:57:23.000000000 +0100

 +++ strace-4.5.16/net.c	2007-07-14 19:29:36.000000000 +0200

 @@ -800,8 +800,12 @@

  #ifdef SOL_PACKET

  static const struct xlat sockpacketoptions[] = {

 +#if defined(PACKET_ADD_MEMBERSHIP)

  	{ PACKET_ADD_MEMBERSHIP,	"PACKET_ADD_MEMBERSHIP"	},

 +#endif

 +#if defined(PACKET_DROP_MEMBERSHIP)

  	{ PACKET_DROP_MEMBERSHIP,	"PACKET_DROP_MEMBERSHIP"},

 +#endif

  #if defined(PACKET_RECV_OUTPUT)

  	{ PACKET_RECV_OUTPUT,		"PACKET_RECV_OUTPUT"	},

  #endif