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>

 Fix for psim build failure due to -lz

 See http://sourceware.org/bugzilla/show_bug.cgi?id=12202

 --- gdb-7.2.orig/sim/ppc/Makefile.in

 +++ gdb-7.2/sim/ppc/Makefile.in

 @@ -551,7 +551,7 @@

  PACKAGE_OBJ = @sim_pk_obj@

 -psim: $(TARGETLIB) main.o $(LIBIBERTY_LIB) $(BFD_LIB) $(LIBS) $(LIBINTL_DEP)

 +psim: $(TARGETLIB) main.o $(LIBIBERTY_LIB) $(BFD_LIB) $(LIBINTL_DEP)

  	$(CC) $(CFLAGS) $(SIM_CFLAGS) $(LDFLAGS) -o psim$(EXEEXT) main.o $(TARGETLIB) $(BFD_LIB) $(LIBINTL) $(LIBIBERTY_LIB) $(LIBS)

  run: psim