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 powerpc-specific values

 CT_DoArchTupleValues () {

     # The architecture part of the tuple, override only for 64-bit

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

         CT_TARGET_ARCH="powerpc64"

     fi

     # Only override values when ABI is not the default

     case "${CT_ARCH_powerpc_ABI}" in

         eabi)

             # EABI is only for bare-metal, so libc ∈ [none,newlib]

             CT_TARGET_SYS="eabi"

             ;;

         spe)

             case "${CT_LIBC}" in

                 none|newlib)    CT_TARGET_SYS="spe";;

                 *glibc)         CT_TARGET_SYS="gnuspe";;

                 uClibc)         CT_TARGET_SYS="uclibcgnuspe";;

             esac

             ;;

     esac

     # Add extra flags for SPE if needed

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

         CT_ARCH_TARGET_CFLAGS="-mabi=spe -mspe"

         CT_ARCH_CC_CORE_EXTRA_CONFIG="--enable-e500_double"

         CT_ARCH_CC_EXTRA_CONFIG="--enable-e500_double"

     fi

 }