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>

 Index: ltrace-0.5.3/configure

 ===================================================================

 --- ltrace-0.5.3/configure

 +++ ltrace-0.5.3/configure

 @@ -78,7 +78,10 @@ fi

  rm -f conftest.c a.out

  CFLAGS='-g -O2'

 -LIBS='-lelf -lsupc++ -liberty '

 +LIBS='-lelf'

 +[ -n "$HAVE_LIBSUPC__" ] && LIBS="$LIBS -lsupc++"

 +[ -n "$HAVE_LIBIBERTY" ] && LIBS="$LIBS -liberty"

 +

  INSTALL='install -c'

  iquote='-iquote '

  iquoteend=''