binman: Use the fake SPL/TPL only if requested

binman: Use the fake SPL/TPL only if requested

At present we always use the main devicetree for SPL/TPL as well when
setting up the state. But this it not needed if there is a real devicetree
for SPL or TPL. In fact it confuses things since we cannot distinguish
between one being provided and using the fake one.

Update the code to create the fakes only when requested. Put the mapping
in a constant so we can use it elsewhere.

Rename 'other_fname' to 'fname' while we are here since there is nothing
'other' about it.

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