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>

 # Compute x86-specific values

 # This one really needs a little love! :-(

 CT_DoArchTupleValues() {

     # Override the architecture part of the tuple:

     if [ "${CT_ARCH_64}" = "y" ]; then

         CT_TARGET_ARCH=x86_64

     else

         arch="${CT_ARCH_ARCH}"

         [ -z "${arch}" ] && arch="${CT_ARCH_TUNE}"

         case "${arch}" in

             "")                           CT_TARGET_ARCH=i386;;

             i386|i486|i586|i686)          CT_TARGET_ARCH="${arch}";;

             winchip*)                     CT_TARGET_ARCH=i486;;

             pentium|pentium-mmx|c3*)      CT_TARGET_ARCH=i586;;

             pentiumpro|pentium*|athlon*)  CT_TARGET_ARCH=i686;;

             prescott)                     CT_TARGET_ARCH=i686;;

             *)                            CT_TARGET_ARCH=i586;;

         esac

     fi

 }