binman: Rename 'position' to 'offset'

binman: Rename 'position' to 'offset'

After some thought, I believe there is an unfortunate naming flaw in
binman. Entries have a position and size, but now that we support
hierarchical sections it is unclear whether a position should be an
absolute position within the image, or a relative position within its
parent section.

At present 'position' actually means the relative position. This indicates
a need for an 'image position' for code that wants to find the location of
an entry without having to do calculations back through parents to
discover this image position.

A better name for the current 'position' or 'pos' is 'offset'. It is not
always an absolute position, but it is always an offset from its parent
offset.

It is unfortunate to rename this concept now, 18 months after binman was
introduced. However I believe it is the right thing to do. The impact is
mostly limited to binman itself and a few changes to in-tree users to
binman:

   tegra
   sunxi
   x86

The change makes old binman definitions (e.g. downstream or out-of-tree)
incompatible if they use the 'pos = <...>' property. Later work will
adjust binman to generate an error when it is used.

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