Normally x86 platforms use the end-at-4gb option. This currently produces
an FMAP with positions which have a large offset. The use of end-at-4gb is
a useful convenience within binman, but we don't really want to export
a map with these offsets.

Fix this by subtracting the 'skip at start' parameter.

Also put the code which convers names to fmap format, for clarity.
Signed-off-by: NSimon Glass <sjg@chromium.org>