scripts: west sign: move hex and bin default locations

When signing binaries from multiple build directories, it is
inconvenient to have to specify the output file locations by hand each
time. It's also a little weird that they're not next to zephyr.bin and
zephyr.hex.

Move them to the build directory, next to their unsigned variants.

Suggested by Piotr Mienkowski.

Signed-off-by: Marti Bolivar <marti.bolivar@nordicsemi.no>