setup_loop: simplify and fix dependency on kernel defaults

This removes some of the options that are the same for every caller. We now use losetup -f to detect free loop devices. This has the advantage that this also works when the kernel is compiled with: CONFIG_BLK_DEV_LOOP_MIN_COUNT=0

Busybox seems to have stable support for -f since 2013 (https://github.com/brgl/busybox/commit/4928e9f7d0a17898300f20567a331417fafc16c1), so I think it is safe to depend on it.

@bluca can you have a look at this?

approved this merge request

merged

mentioned in commit 4364b220