* The system image size is now calculated as: root size - home size.
* New function in `pmb/helpers/other.py`: `folder_size()`, with a
testcase.
* Instead of copying everything to the system image folder, and
deleting the home folder afterwards, do not copy the home folder
in the first place.
* Added `pmbootstrap -s` to skip generating the initramfs for faster
debugging.
* Set the default value in the "are you sure, that your partition has
at least..." to "y", so we can run `yes '' | pmbootstrap install`
to make it run through the whole installation process.
* Increase full size to 120%, boot partition gets 15 MB free space now
