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>

 do not bother running ldconfig on DESTDIR.  it wants to write the temp cache

 file outside of the chroot.  doesnt matter anyways as we wont use the cache

 results (portage will rebuild cache), so running ldconfig is simply a waste

 of time.

 diff -durN glibc-2.10.1.orig/Makefile glibc-2.10.1/Makefile

 --- glibc-2.10.1.orig/Makefile	2009-04-23 20:23:03.000000000 +0200

 +++ glibc-2.10.1/Makefile	2009-11-13 00:49:49.000000000 +0100

 @@ -116,6 +116,7 @@

  	rm -f $(symbolic-link-list)

  install:

 +dont-bother-with-destdir:

  	-test ! -x $(common-objpfx)elf/ldconfig || LC_ALL=C LANGUAGE=C \

  	  $(common-objpfx)elf/ldconfig $(addprefix -r ,$(install_root)) \

  				       $(slibdir) $(libdir)