System config: split static-dev device table setting into seperate option

As discussed here:

http://lists.busybox.net/pipermail/buildroot/2011-May/043251.html



Add BR2_ROOTFS_STATIC_DEVICE_TABLE for the extra device table file(s)
to create device nodes in /dev, rather than complicated logic in
BR2_ROOTFS_DEVICE_TABLE, making it complicated to move between static
and dynamic modes.

Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>