Allow several device tables and split in two parts our device table

This allows to have a device table for all directories/files and
another device table for the device files themselves. Both are needed
for static /dev, but only the first one is needed when
devtmpfs/mdev/udev are used.

We take this opportunity to move the documentation of the device table
format in a common location, package/makedevs/README.

[Peter: simplify code slightly, fix indentation]
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>