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>

 --- uClibc-0.9.30.orig/extra/scripts/install_headers.sh	2008-10-10 15:50:38.000000000 +0200

 +++ uClibc-0.9.30/extra/scripts/install_headers.sh	2011-08-21 09:49:03.000000000 +0200

 @@ -33,7 +33,7 @@

  (

  # We must cd, or else we'll prepend "$1" to filenames!

  cd "$1" || exit 1

 -find ! -name '.' -a ! -path '*/.*' | sed -e 's/^\.\///' -e '/^config\//d' \

 +find . ! -name '.' -a ! -path '*/.*' | sed -e 's/^\.\///' -e '/^config\//d' \

  	-e '/^config$/d'

  ) | \

  (