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 -ru ltrace-0.5.3.org/Makefile.in ltrace-0.5.3/Makefile.in

 --- ltrace-0.5.3.org/Makefile.in	2011-08-21 18:55:15.000000000 +0200

 +++ ltrace-0.5.3/Makefile.in	2011-08-21 18:40:53.000000000 +0200

 @@ -15,6 +15,7 @@

  mandir		= @mandir@

  docdir		= $(prefix)/share/doc/ltrace

 +AR		=	@AR@

  CC		=	@CC@

  CFLAGS		=	-Wall @CFLAGS@

  CPPFLAGS	=	-iquote $(TOPDIR) -iquote $(TOPDIR)/sysdeps/$(OS) -DSYSCONFDIR=\"$(sysconfdir)\" @CPPFLAGS@

 diff -ru ltrace-0.5.3.org/configure ltrace-0.5.3/configure

 --- ltrace-0.5.3.org/configure	2011-08-21 18:55:15.000000000 +0200

 +++ ltrace-0.5.3/configure	2011-08-21 18:54:46.000000000 +0200

 @@ -113,7 +117,7 @@

  #

  # Makefile.in -> Makefile

  #

 -x_subst_vars='PACKAGE_VERSION HOST HOST_OS INSTALL CC CPPFLAGS CFLAGS LDFLAGS LIBS iquote iquoteend prefix sysconfdir mandir docdir'

 +x_subst_vars='PACKAGE_VERSION HOST HOST_OS INSTALL AR CC CPPFLAGS CFLAGS LDFLAGS LIBS iquote iquoteend prefix sysconfdir mandir docdir'

  for i in $x_subst_vars

  do