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>

 YEM-20070519:

 Patch from Bernhard Fischer <rep.dot.nop@gmail.com> on the uClibc mailing

 list ( http://www.uclibc.org/lists/uclibc/2008-January/018940.html ) above

 the 400-bits_sysnum_h.patch.

 Index: uClibc/Makefile.in

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

 --- uClibc/Makefile.in	(revision 18651)

 +++ uClibc/Makefile.in	(working copy)

 @@ -114,7 +116,7 @@ install: install_runtime install_dev

  RUNTIME_PREFIX_LIB_FROM_DEVEL_PREFIX_LIB=$(shell $(top_srcdir)extra/scripts/relative_path.sh $(DEVEL_PREFIX)lib $(RUNTIME_PREFIX)lib)

  # Installs header files.

 -install_headers:

 +install_headers: headers

  	$(INSTALL) -d $(PREFIX)$(DEVEL_PREFIX)include

  	printf ".svn\n.cvsignore\nCVS\n" > tar_exclude ; \

  	$(TAR) -chf - -X tar_exclude include \