binman: Update entry.SetOffsetSize to be optional

binman: Update entry.SetOffsetSize to be optional

At present this function always sets both the offset and the size of
entries. But in some cases we want to set only one or the other, for
example with the forthcoming ifwi entry, where we only set the offset.
Update the function to handle this.

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