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>

docs/manual/customize-rootfs.txt

@@ -11,8 +11,8 @@ There are a few ways to customize the resulting target filesystem:
   rebuild your toolchain and tools, these changes will be lost.
 
 * Create your own 'target skeleton'. You can start with the default
-  skeleton available under +fs/skeleton+ and then customize it to suit
-  your needs. The +BR2_ROOTFS_SKELETON_CUSTOM+ and
+  skeleton available under +system/skeleton+ and then customize it to
+  suit your needs. The +BR2_ROOTFS_SKELETON_CUSTOM+ and
   +BR2_ROOTFS_SKELETON_CUSTOM_PATH+ will allow you to specify the
   location of your custom skeleton. At build time, the contents of the
   skeleton are copied to output/target before any package