Fix #363: Omit /home/user when calculating system image size (#389)

* 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

pmb/chroot/initfs.py

@@ -25,6 +25,11 @@ import pmb.helpers.cli
 
 
 def build(args, flavor, suffix):
+    # Bail out when '-s' is set
+    if args.skip_initfs:
+        logging.info("NOTE: Skipped initramfs generation (-s)!")
+        return
+
     # Update mkinitfs and hooks
     pmb.chroot.apk.install(args, ["postmarketos-mkinitfs"], suffix)
     pmb.chroot.initfs_hooks.update(args, suffix)