I've realized that truncate is provided by busybox anyway, so no
need to install coreutils for this.
This should fix creating system image files on ZFS, where fallocate
is not implemented.
* Validate all inputs from `pmbootstrap init`
* Add a new `confirm()` function, that validates input of yes/no
questions properly
* All questions loop until they have a valid answer now
* The check for valid sdcard paths has been removed (because it
doesn't make sense for /dev/sd*, this might as well be a real
harddrive)
* New check: abort if any partition of the sdcard is mounted
* Automatically detect if the partitions are called "p1", "p2" etc.
or just "1", "2" etc. (/dev/sda1 vs. /dev/mmcblk0p1)
* pmb.helpers.mount.ismount(): check the source (new) and the target
path in /proc/mounts
