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>

 diff -durN gcc-4.2.3.orig/gcc/config/arm/linux-eabi.h gcc-4.2.3/gcc/config/arm/linux-eabi.h

 --- gcc-4.2.3.orig/gcc/config/arm/linux-eabi.h	2007-09-01 17:28:30.000000000 +0200

 +++ gcc-4.2.3/gcc/config/arm/linux-eabi.h	2008-05-25 23:47:36.000000000 +0200

 @@ -47,7 +47,8 @@

  #define SUBTARGET_CPU_DEFAULT TARGET_CPU_arm10tdmi

  #undef SUBTARGET_EXTRA_LINK_SPEC

 -#define SUBTARGET_EXTRA_LINK_SPEC " -m armelf_linux_eabi"

 +#define SUBTARGET_EXTRA_LINK_SPEC	\

 +	" %{mlittle-endian:-m armelf_linux_eabi} %{mbig-endian:-m armelfb_linux_eabi}"

  /* Use ld-linux.so.3 so that it will be possible to run "classic"

     GNU/Linux binaries on an EABI system.  */