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/helpers/other.py

@@ -17,6 +17,21 @@ You should have received a copy of the GNU General Public License
 along with pmbootstrap.  If not, see <http://www.gnu.org/licenses/>.
 """
 import os
+import pmb.helpers.run
+
+
+def folder_size(args, path):
+    """
+    Run `du` to calculate the size of a folder (this is less code and
+    faster than doing the same task in pure Python).
+
+    :returns: folder size in bytes
+    """
+    output = pmb.helpers.run.root(args, ["du", "--summarize",
+                                         "--block-size=1",
+                                         path], return_stdout=True)
+    ret = int(output.split("\t")[0])
+    return ret
 
 
 def check_grsec(args):