binman: Honour the skip-at-start property more faithfully

binman: Honour the skip-at-start property more faithfully

A discussion on the mailing list about dealing with block offsets and
binman symbols made me think that something is wrong with how Binman
deals with the skip-at-start property.

The feature was originally designed to handle x86 ROMs, which are mapped
at the top of the address space. That seemed too specific, whereas
skipping some space at the start seemed more generally useful.

It has proved useful. For example, rockchip images start at block 64,
so a skip-at-start of 0x8000 deals with this.

But it doesn't actually work correctly, since the image_pos value does
not give the actual position on the media.

Fix this and update the documentation, moving it into the 'section'
section.

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