aufs: new kernel extension

Signed-off-by: Christian Stewart <christian@paral.in>
[Atul:
 - Removed the duplicate conditional block.
 - Updated the license to GPLv2.
 - Removed the visibilty of package from menuconfig.
 - Removed dependencies.
 - Removed the comment.
 - Changed the name of variable from BR2_PACKAGE_AUFS_STANDALONE_VERSION
   to BR2_LINUX_KERNEL_EXT_AUFS_VERSION.
 - Removed the AUFS_INSTALL_STAGING  and AUFS_INSTALL_TARGET  variables.
 - Removed the BR2_PACKAGE_AUFS_3X and BR2_PACKAGE_AUFS_4X variables.]
Signed-off-by: Atul Singh <atul.singh.mandla@rockwellcollins.com>
[yann.morin.1998@free.fr:
  - do not fail on version check if aufs ext is disabled
  - check for empty version
  - squash aufs package and linux extension in one patch
  - fail if the kernel already has aufs support
  - simplify handling of version]
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>
[Thomas:
 - Fix the apply patch logic, it was using a non-existent
   AUFS_VERSION_MAJOR variable. BR2_PACKAGE_AUFS_SERIES is used
   instead.]
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>