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>

 Original patch from: ../4.3.2/270-missing-execinfo_h.patch

 -= BEGIN original header =-

 Original patch from gentoo: gentoo/src/patchsets/gcc/4.3.1/uclibc/90_all_301-missing-execinfo_h.patch

 -= END original header =-

 diff -durN gcc-4.3.3.orig/boehm-gc/include/gc.h gcc-4.3.3/boehm-gc/include/gc.h

 --- gcc-4.3.3.orig/boehm-gc/include/gc.h	2007-04-23 23:10:09.000000000 +0200

 +++ gcc-4.3.3/boehm-gc/include/gc.h	2009-01-27 22:25:37.000000000 +0100

 @@ -503,7 +503,7 @@

  #if defined(__linux__) || defined(__GLIBC__)

  # include <features.h>

  # if (__GLIBC__ == 2 && __GLIBC_MINOR__ >= 1 || __GLIBC__ > 2) \

 -     && !defined(__ia64__)

 +     && !defined(__ia64__) && !defined(__UCLIBC__)

  #   ifndef GC_HAVE_BUILTIN_BACKTRACE

  #     define GC_HAVE_BUILTIN_BACKTRACE

  #   endif