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>

 #!@@CT_bash@@

 # What we need:

 #  - the .config file

 # Parse the tools' paths configuration

 . "${CT_LIB_DIR}/paths.sh"

 # We'll need the stdout later, save it

 exec 7>&1

 # Parse the common functions

 . "${CT_LIB_DIR}/scripts/functions"

 # Don't care about any log file

 exec >/dev/null

 rm -f "${tmp_log_file}"

 # Parse the configuration file

 . .config.2

 # Parse architecture and kernel specific functions

 . "${CT_LIB_DIR}/scripts/build/arch/${CT_ARCH}.sh"

 . "${CT_LIB_DIR}/scripts/build/kernel/${CT_KERNEL}.sh"

 # Build CT_TARGET

 CT_DoBuildTargetTuple

 # All this for this single echo... :-( Sigh, I'll have to re-arrange things...

 echo "${CT_TARGET}" >&7