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>

 timezone data has been split into the package sys-libs/timezone-data

 diff -durN glibc-2.12.1.orig/Makeconfig glibc-2.12.1/Makeconfig

 --- glibc-2.12.1.orig/Makeconfig	2009-11-13 00:50:13.000000000 +0100

 +++ glibc-2.12.1/Makeconfig	2009-11-13 00:50:41.000000000 +0100

 @@ -944,7 +944,7 @@

  	      stdlib stdio-common libio malloc string wcsmbs time dirent    \

  	      grp pwd posix io termios resource misc socket sysvipc gmon    \

  	      gnulib iconv iconvdata wctype manual shadow gshadow po argp   \

 -	      crypt nss localedata timezone rt conform debug		    \

 +	      crypt nss localedata rt conform debug		    \

  	      $(add-on-subdirs) $(dlfcn) $(binfmt-subdir)

  ifndef avoid-generated