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-3.4.6.orig/libstdc++-v3/fragment.am gcc-3.4.6/libstdc++-v3/fragment.am

 --- gcc-3.4.6.orig/libstdc++-v3/fragment.am	2003-08-27 02:38:28.000000000 +0200

 +++ gcc-3.4.6/libstdc++-v3/fragment.am	2007-08-15 22:59:09.000000000 +0200

 @@ -18,7 +18,7 @@

  	$(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 -durN gcc-3.4.6.orig/libstdc++-v3/libmath/Makefile.am gcc-3.4.6/libstdc++-v3/libmath/Makefile.am

 --- gcc-3.4.6.orig/libstdc++-v3/libmath/Makefile.am	2003-08-27 23:29:43.000000000 +0200

 +++ gcc-3.4.6/libstdc++-v3/libmath/Makefile.am	2007-08-15 22:59:09.000000000 +0200

 @@ -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