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 glibc-2.7.orig/Makerules glibc-2.7/Makerules

 --- glibc-2.7.orig/Makerules	2007-08-26 04:18:03.000000000 +0200

 +++ glibc-2.7/Makerules	2008-07-28 15:17:27.000000000 +0200

 @@ -1203,6 +1203,9 @@

  # The include magic above causes those files to use this variable for flags.

  CPPFLAGS-nonlib = -DNOT_IN_libc=1

 +ifneq ($(OBJDUMP_FOR_HOST),)

 +OBJDUMP = $(OBJDUMP_FOR_HOST)

 +endif

  ifeq ($(versioning),yes)

  # Generate normalized lists of symbols, versions, and data sizes.