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>

 This patch is courtesy of OpenEmbedded, by Khem Raj <raj.khem@gmail.com>

 http://cgit.openembedded.org/cgit.cgi/openembedded/commit/?id=b85b779688f0abc389e3c25d06b54de8b86385c1

 Index: ltrace-0.5.3/Makefile.in

 ===================================================================

 --- ltrace-0.5.3.orig/Makefile.in	2009-10-23 22:06:08.130304691 -0700

 +++ ltrace-0.5.3/Makefile.in	2009-10-23 22:06:53.160369614 -0700

 @@ -39,6 +39,8 @@ ltrace:		main.o libltrace.a

  libltrace.a: 	sysdeps/sysdep.o $(OBJ)

  		$(AR) rcv $@ $^

 +$(OBJ):		sysdeps/sysdep.o

 +

  sysdeps/sysdep.o: dummy

  		$(MAKE) -C sysdeps/$(OS)