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>

 # Wrapper to build the debug facilities

 # List all debug facilities, and parse their scripts

 CT_DEBUG_FACILITY_LIST=

 for f in "${CT_LIB_DIR}/scripts/build/debug/"*.sh; do

     _f="$(basename "${f}" .sh)"

     _f="${_f#???-}"

     __f="CT_DEBUG_${_f}"

     if [ "${!__f}" = "y" ]; then

         CT_DoLog DEBUG "Enabling debug '${_f}'"

         . "${f}"

         CT_DEBUG_FACILITY_LIST="${CT_DEBUG_FACILITY_LIST} ${_f}"

     else

         CT_DoLog DEBUG "Disabling debug '${_f}'"

     fi

 done

 # Download the debug facilities

 do_debug_get() {

     for f in ${CT_DEBUG_FACILITY_LIST}; do

         do_debug_${f}_get

     done

 }

 # Extract and patch the debug facilities

 do_debug_extract() {

     for f in ${CT_DEBUG_FACILITY_LIST}; do

         do_debug_${f}_extract

     done

 }

 # Build the debug facilities

 do_debug() {

     for f in ${CT_DEBUG_FACILITY_LIST}; do

         do_debug_${f}_build

     done

 }