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

@@ -39,7 +40,7 @@

 		$(CC) $(LDFLAGS) $^ $(LIBS) -o $@

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

-		$(AR) rcv $@ $^

+		$(AR) rscv $@ $^

 $(OBJ):		sysdeps/sysdep.o