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>

 diff -durN gcc-4.2.1.orig/gcc/Makefile.in gcc-4.2.1/gcc/Makefile.in

 --- gcc-4.2.1.orig/gcc/Makefile.in	2007-05-31 17:37:38.000000000 +0200

 +++ gcc-4.2.1/gcc/Makefile.in	2007-08-03 20:36:14.000000000 +0200

 @@ -2660,7 +2660,7 @@

  # FIXME: writing proper dependencies for this is a *LOT* of work.

  libbackend.o : $(OBJS-common:.o=.c) $(out_file) \

    insn-config.h insn-flags.h insn-codes.h insn-constants.h \

 -  insn-attr.h  $(DATESTAMP) $(BASEVER) $(DEVPHASE)

 +  insn-attr.h  $(DATESTAMP) $(BASEVER) $(DEVPHASE) gcov-iov.h

  	$(CC) $(ALL_CFLAGS) $(ALL_CPPFLAGS) \

  	  -DTARGET_NAME=\"$(target_noncanonical)\" \

  	  -DLOCALEDIR=\"$(localedir)\" \