skeleton-custom: install /bin, /lib, and /sbin

skeleton-custom: install /bin, /lib, and /sbin

skeleton-custom does not install the required /bin, /lib and /sbin
directories (or symlinks), which may result in an imcomplete tree, The
user could add the required directories/symlinks to the skeleton but
they may be invalid, depending on the state of BR2_ROOTFS_MERGED_USR.

Steps to reproduce:

- Enable BR2_ROOTFS_MERGED_USR and BR2_INIT_SYSTEMD
- Set BR2_ROOTFS_SKELETON_CUSTOM_PATH to "system/skeleton"
- Run "make skeleton"
- target/{bin.lib,sbin} will not exist

Add calls to SYSTEM_USR_SYMLINKS_OR_DIRS to INSTALL_TARGET_CMDS and
INSTALL_STAGING_CMDS, so the required directories or symlinks are
created.

Add a paragraph to the documentation clarifying that custom skeletons
don't need to contain /bin, /lib or /sbin and must not contain them when
BR2_ROOTFS_MERGED_USR is enabled.

Signed-off-by: Carlos Santos <casantos@datacom.ind.br>
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>