New top-level directory: system

This directory groups the following elements:
 * the default root filesystem skeleton
 * the default device tables
 * the Config.in options for system configuration (UART port for
   getty, system hostname, etc.)
 * the make rules to apply the system configuration options

Even though the skeleton and device tables could have lived in fs/, it
would have been strange to have the UART, system hostname and other
related options into fs/. A new system/ directory makes more sense.

As a consequence, this patch also removes target/Makefile.in, which
has become useless in the process.

[Peter: fixup TARGET_SKELETON settings / documentation to match]
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Acked-by: Yann E. MORIN <yann.morin.1998@free.fr>
Acked-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>