Change file_loadraw() from static to public.  Change the order of its
arguments from type,filename to filename,type to be consistant with other
public file_whatever() functions, and change it to return a pointer to
the preloaded_file struct describing the file.  Adjust existing callers.
ext2fs: fully enable ext4 read-only support.

The ext4 developers tend to tag Ext4-specific flags as
"incompatible" even when such features are not relevant for
read-only support.  This is a consequence of the process
though which this filesystem is implemented without design
and the fact that some new features are not extensible to
ext2/3.

Organize the features according to what we support and sort
them so that we can now read-only mount filesystems with
some features that may be found in newly formatted ext4 fs.

Submitted by:	Zheng Liu
Reviewed by:	pfg
MFC after:	5 days
Add a feature for automatically finding and loading a dtb file by name.
The name is taken from the u-boot env vars fdtfile or fdt_file.  If the
name isn't fully-qualified a search is done in module_path locations.

The search order for a usable dtb in fdt_setup_fdtp() is now

 - A dtb loaded with an explicit "load -t dtb" command.
 - A dtb already loaded into memory somehow[*] and pointed to by fdt_to_load.
 - A dtb in the memory pointed to by the u-boot env vars fdtaddr or fdt_addr.
 - A file named by the u-boot env vars fdtfile or fdt_file.
 - A static dtb compiled into the kernel.

* Presumably by some arch-specific command or code.