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>

 --- dmalloc-5.5.2.orig/configure	2008-06-17 13:10:09.000000000 +0200

 +++ dmalloc-5.5.2/configure	2008-06-17 13:11:25.000000000 +0200

 @@ -4691,8 +4691,35 @@

  echo "$as_me:$LINENO: checking strdup macro" >&5

  echo $ECHO_N "checking strdup macro... $ECHO_C" >&6

  if test "$cross_compiling" = yes; then

 -  ac_cv_strdup_macro=no

 +  cat >conftest.$ac_ext <<_ACEOF

 +/* confdefs.h */

 +_ACEOF

 +cat confdefs.h >>conftest.$ac_ext

 +cat >>conftest.$ac_ext <<_ACEOF

 +/* end confdefs.h.  */

 +

 +#if HAVE_STDLIB_H

 +#  include <string.h>

 +#endif

 +

 +#ifndef strdup

 +       choke me

 +#endif

 +main() { exit(0); }

 +

 +_ACEOF

 +rm -f conftest.o conftest.obj

 +if { (eval echo "$as_me:$LINENO: \"$ac_compile\"") >&5

 +  (eval $ac_compile) 2>&5

 +  ac_status=$?

 +  echo "$as_me:$LINENO: \$? = $ac_status" >&5

 +  (exit $ac_status); }; then

 +  ac_cv_strdup_macro=yes

 +else

 +  ac_cv_strdup_macro=no

 +fi

 +  

  else

    cat >conftest.$ac_ext <<_ACEOF

  /* confdefs.h.  */