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>

 Enable building a pure soft-float compiler without the need for a software

 floating point library.

 diff -dur gcc-4.0.4.orig/gcc/config.gcc gcc-4.0.4/gcc/config.gcc

 --- gcc-4.0.4.orig/gcc/config.gcc	2007-02-02 19:12:28.000000000 +0100

 +++ gcc-4.0.4/gcc/config.gcc	2007-02-02 19:12:07.000000000 +0100

 @@ -677,7 +677,7 @@

  		tm_defines="TARGET_BIG_ENDIAN_DEFAULT=1 $tm_defines"

  		;;

  	esac

 -	tmake_file="${tmake_file} arm/t-arm arm/t-linux"

 +	tmake_file="${tmake_file} arm/t-arm arm/t-linux arm/t-arm-elf"

  	extra_parts="crtbegin.o crtbeginS.o crtend.o crtendS.o"

  	gnu_ld=yes

  	;;