guess input file format based on file content or file name

Always needing to specify the input file format can be quite
annoying, especially since a dtb is easily detected by its magic.
Looking at the file name extension sounds useful as a hint, too.

Add heuristic file type guessing of the input file format in case
none has been specified on the command line.
The heuristics are as follows (in that order):
- Any issues with opening the file drop back to the current default
behaviour.
- A directory will be treated as the /proc/device-tree type.
- If the first 4 bytes are the DTB magic, assume "dtb".
- If no other test succeeded so far, use a file name based
guessing method: if the filename ends with .dts or .DTS, device tree
source text is assumed, .dtb or .DTB hint at a device tree blob.

For the majority of practical use cases this gets rid of the tedious
-I specification on the command line and simplifies actual typing of
dtc command lines.
Any explicit specification of the input type by using -I still avoids
any guessing, which resembles the current behaviour.

Signed-off-by: Andre Przywara <osp@andrep.de>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>