flashprog.c: Let select_erase_functions() return byte count

Having the total byte count that will be erased helps to calculate
an overall progress state.

Change-Id: Ib4c326d1c98e97325b6ffd67363bbba4f9194678
Signed-off-by: Nico Huber <nico.h@gmx.de>
Reviewed-on: https://review.sourcearcade.org/c/flashprog/+/74730
Reviewed-by: Arthur Heymans <arthur@aheymans.xyz>
1 file changed