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 duma_2_5_12.orig/Makefile duma_2_5_12/Makefile

 --- duma_2_5_12.orig/Makefile	2007-08-17 09:34:25.000000000 +0200

 +++ duma_2_5_12/Makefile	2007-09-07 21:50:04.000000000 +0200

 @@ -93,9 +93,6 @@

  # also define 'WIN32'

  # some defaults:

 -CC=gcc

 -CXX=g++

 -AR=ar

  INSTALL=install

  RM=rm

  RMFORCE=rm -f

 @@ -299,7 +296,7 @@

  createconf$(EXEPOSTFIX): createconf.o

  	- $(RMFORCE) createconf$(EXEPOSTFIX)

 -	$(CC) $(CFLAGS) $(DUMA_OPTIONS) createconf.o -o createconf$(EXEPOSTFIX)

 +	$(HOSTCC) $(HOSTCFLAGS) $(DUMA_OPTIONS) createconf.o -o createconf$(EXEPOSTFIX)

  tstheap$(EXEPOSTFIX): libduma.a tstheap.o

  	- $(RMFORCE) tstheap$(EXEPOSTFIX)

 @@ -360,7 +357,7 @@

  # define rules how to build objects for createconf

  #

  createconf.o:

 -	$(CC) $(CFLAGS) $(DUMA_OPTIONS) -c createconf.c -o $@

 +	$(HOSTCC) $(HOSTCFLAGS) $(DUMA_OPTIONS) -c createconf.c -o $@

  #