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_EXPERIMENTAL=y

 CT_LOCAL_TARBALLS_DIR="${HOME}/src"

 CT_SAVE_TARBALLS=y

 CT_LOG_EXTRA=y

 CT_ARCH_64=y

 CT_ARCH_powerpc=y

 CT_KERNEL_linux=y

 CT_KERNEL_V_2_6_31_14=y

 CT_BINUTILS_V_2_20_1a=y

 CT_BINUTILS_FOR_TARGET=y

 CT_CC_V_4_4_5=y

 CT_CC_LANG_FORTRAN=y

 CT_CC_LANG_CXX=y

 CT_CC_LANG_JAVA=y

 CT_LIBC_glibc=y

 CT_LIBC_GLIBC_V_2_9=y

 # CT_LIBC_GLIBC_FORCE_UNWIND is not set

 CT_DEBUG_dmalloc=y

 CT_DEBUG_duma=y

 CT_DEBUG_gdb=y

 CT_GDB_NATIVE=y

 CT_GDB_V_6_8a=y

 CT_DEBUG_strace=y

 CT_STRACE_V_4_5_19=y

 CT_GMP_V_5_0_1=y

 CT_MPFR_V_3_0_0=y

 CT_PPL_V_0_10_2=y

 CT_CLOOG_V_0_15_10=y