binman: Use the actual contents in CheckSize()

At present this function adds up the total size of entries to work out the
size of a section's contents. With compression this is no-longer enough.

We may as well bite the bullet and build the section contents instead.
Call _BuildSectionData() to get the (possibly compressed) contents and
GetPaddedData() to get the same but with padding added.

Note that this is inefficient since the section contents is calculated
twice. Future work will improve this.

This affects testPackOverlapMap() since the error is reported with a
different section size now (enough to hold the contents). Update that at
the same time.

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