binman: Refactor much of the image code into 'section'

We want to support multiple sections within a single image. To do this,
move most of the Image class implementation into a new Section class. An
Image contains only a single Section, but at some point we will support
a new 'section' entry, thus allowing Sections within Sections.

Use the name 'bsection' for the module so we can use 'section' for the
etype module.

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