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>

 This patch is courtesy of OpenEmbedded, by Khem Raj <raj.khem@gmail.com>

 http://cgit.openembedded.org/cgit.cgi/openembedded/commit/?id=b85b779688f0abc389e3c25d06b54de8b86385c1

 Index: ltrace-0.5.3/sysdeps/linux-gnu/mipsel/trace.c

 ===================================================================

 --- ltrace-0.5.3.orig/sysdeps/linux-gnu/mipsel/trace.c	2009-10-23 23:15:59.822783128 -0700

 +++ ltrace-0.5.3/sysdeps/linux-gnu/mipsel/trace.c	2009-10-23 23:16:10.441268788 -0700

 @@ -127,7 +127,6 @@ gimme_arg(enum tof type, Process *proc, 

  			return ret;

  		} else {

  			// If we need this, I think we can look at [sp+16] for arg_num==4.

 -			CP;

  			return 0;

  		}

  	}