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 gentoo: gentoo/src/patchsets/gcc/4.3.1/uclibc/90_all_303-c99-complex-ugly-hack.patch

 diff -durN gcc-4.3.2.orig/libstdc++-v3/configure gcc-4.3.2/libstdc++-v3/configure

 --- gcc-4.3.2.orig/libstdc++-v3/configure	2008-07-07 21:49:54.000000000 +0200

 +++ gcc-4.3.2/libstdc++-v3/configure	2008-09-23 19:45:27.000000000 +0200

 @@ -38935,6 +38935,9 @@

  cat >>conftest.$ac_ext <<_ACEOF

  /* end confdefs.h.  */

  #include <complex.h>

 +#ifdef __UCLIBC__

 +#error ugly hack to make sure configure test fails here for cross until uClibc supports the complex funcs

 +#endif

  int

  main ()

  {