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 -urN gcc-4.0.0-100/libstdc++-v3/fragment.am gcc-4.0.0/libstdc++-v3/fragment.am

 --- gcc-4.0.0-100/libstdc++-v3/fragment.am	2004-10-25 15:32:40.000000000 -0500

 +++ gcc-4.0.0/libstdc++-v3/fragment.am	2005-04-28 21:48:43.000000000 -0500

 @@ -18,5 +18,5 @@

  	$(WARN_FLAGS) $(WERROR) -fdiagnostics-show-location=once

  # -I/-D flags to pass when compiling.

 -AM_CPPFLAGS = $(GLIBCXX_INCLUDES)

 +AM_CPPFLAGS = $(GLIBCXX_INCLUDES) -I$(toplevel_srcdir)/include

 diff -urN gcc-4.0.0-100/libstdc++-v3/libmath/Makefile.am gcc-4.0.0/libstdc++-v3/libmath/Makefile.am

 --- gcc-4.0.0-100/libstdc++-v3/libmath/Makefile.am	2003-08-27 16:29:42.000000000 -0500

 +++ gcc-4.0.0/libstdc++-v3/libmath/Makefile.am	2005-04-28 21:48:43.000000000 -0500

 @@ -32,7 +32,7 @@

  libmath_la_SOURCES = stubs.c

 -AM_CPPFLAGS = $(CANADIAN_INCLUDES)

 +AM_CPPFLAGS = $(CANADIAN_INCLUDES) -I$(toplevel_srcdir)/include

  # Only compiling "C" sources in this directory.

  LIBTOOL = @LIBTOOL@ --tag CC