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>

many ports hit this warning:

../sysdeps/unix/sysv/linux/check_native.c:46: warning: implicit declaration of function 'memset'

../sysdeps/unix/sysv/linux/check_native.c:46: warning: implicit declaration of function 'memset'

snipped from suse

diff -durN glibc-2.13.orig/sysdeps/unix/sysv/linux/check_native.c glibc-2.13/sysdeps/unix/sysv/linux/check_native.c

--- glibc-2.13.orig/sysdeps/unix/sysv/linux/check_native.c	2007-11-24 04:12:17.000000000 +0100

+++ glibc-2.13/sysdeps/unix/sysv/linux/check_native.c	2009-11-13 00:50:11.000000000 +0100

@@ -23,6 +23,7 @@

 #include <stddef.h>

 #include <stdint.h>

 #include <stdlib.h>

+#include <string.h>

 #include <time.h>

 #include <unistd.h>

 #include <net/if.h>