skeleton: New package

skeleton: New package

Create a proper package for the skeleton.

The main Makefile is modified to remove the skeleton support.
The 'dirs' target, will create the $(TARGET_DIR).
The file 'output/target/.root' doesn't exists anymore, as there's no
Make rule to statisfy.

The infrastructure are modified to filter host-skeleton.
It's needed becauses the host-dependencies are derived from the
dependencies of the target package where 'host-' is preprended to the
depedency name.

In the pkg-generic we add skeleton as a dependency to every package.

The whole system/system.mk is now removed at the profit of
package/skeleton/skeleton.mk

[Thomas:
  - rebase on top of master and fix some minor conflicts
  - remove the 'select BR2_PACKAGE_SKELETON' in
    BR2_ROOTFS_SKELETON_DEFAULT and BR2_ROOTFS_SKELETON_CUSTOM, since
    anyway the skeleton package is always enabled.
  - fixup a few mistakes in the getty handling due to misnamed
    variables.]

Signed-off-by: Maxime Hadjinlian <maxime.hadjinlian@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>