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_LOCAL_TARBALLS_DIR="${HOME}/src"

 CT_SAVE_TARBALLS=y

 CT_LOG_EXTRA=y

 CT_ARCH_blackfin=y

 CT_KERNEL_linux=y

 CT_KERNEL_V_2_6_33_20=y

 CT_ARCH_BINFMT_FDPIC=y

 CT_BINUTILS_V_2_20_1a=y

 CT_CC_V_4_3_4=y

 CT_LIBC_UCLIBC_V_0_9_30_2=y

 CT_LIBC_UCLIBC_CONFIG_FILE="${CT_LIB_DIR}/samples/${CT_TARGET}/${CT_LIBC}-${CT_LIBC_VERSION}.config"

 CT_GMP_V_4_3_2=y

 CT_MPFR_V_2_4_2=y