Assumed to be a remnant of early development lead to a recent transition
of "start_sector" from being represented as a string to an integer. But
it turns out that "start_sector" might be an expression, e.g. to write
something at the end of the disk.

So transition back to carry "start_sector" as a string.

Fixes: b6e0ea31

 ("program: Make start_sector unsigned in")
Reported-by: Julien Robin
Signed-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>