binman: Allow extracting a file in an alternative format

In some cases entries encapsulate other data and it is useful to access
the data within. An example is the fdtmap which consists of a 16-byte
header, followed by a devicetree.

Provide an option to specify an alternative format when extracting files.
In the case of fdtmap, this is 'fdt', which produces an FDT file which can
be viewed with fdtdump.

Signed-off-by: Simon Glass <sjg@chromium.org>