I've replaced all instances in the code of `os.path.abspath`
with `os.path.realpath`, as this does the same as `abspath`
plus resolving symlinks.
See also: https://stackoverflow.com/a/40311142
* Refactored `umount_all` to get the list of folders to be umounted from
`umount_all_list`, so we can test that function in a test case.
* Adjusted `umount_all_list` to return the deep folder levels first
* Added a testcase for that
* Remove redundant calls to `umount_all()` (which were from a time before
`pmbootstrap` was released, in which failing commands did not cause
`pmbootstrap` to raise an exception)
* 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
