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>

 http://git.frugalware.org/gitweb/gitweb.cgi?p=frugalware-current.git;a=blob_plain;f=source/base/glibc/make-3.82-fix.patch;hb=8217c32ecc2e14962847ba3d8a272eb64a3dba4f

 --- glibc-2.10.1.OLD/manual/Makefile

 +++ glibc-2.10.1.NEW/manual/Makefile

 @@ -232,7 +232,9 @@

  .PHONY: stubs

  stubs: $(objpfx)stubs

  endif

 -$(objpfx)stubs ../po/manual.pot $(objpfx)stamp%:

 +$(objpfx)stubs ../po/manual.pot:

 +	touch $@

 +$(objpfx)stamp%:

  	$(make-target-directory)

  	touch $@