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 ARM-specific values

 CT_DoArchTupleValues() {

     # The architecture part of the tuple:

     CT_TARGET_ARCH="${CT_ARCH}${target_endian_eb}"

     # The system part of the tuple:

     case "${CT_LIBC},${CT_ARCH_ARM_EABI}" in

         *glibc,y)   CT_TARGET_SYS=gnueabi;;

         uClibc,y)   CT_TARGET_SYS=uclibcgnueabi;;

         *,y)        CT_TARGET_SYS=eabi;;

     esac

     # Set the default instruction set mode

     case "${CT_ARCH_ARM_MODE}" in

         arm)    ;;

         thumb)

             CT_ARCH_CC_CORE_EXTRA_CONFIG="--with-mode=thumb"

             CT_ARCH_CC_EXTRA_CONFIG="--with-mode=thumb"

 #            CT_ARCH_TARGET_CFLAGS="-mthumb"

             ;;

     esac

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

         CT_ARCH_TARGET_CFLAGS+=" -mthumb-interwork"

     fi

 }