New init configuration option for free space on the image file

Created by: BrianOtto

I am currently running pmbootstrap in qemu and trying to compile a very large application after it boots up. The 20% extra space that #336 automatically adds is not enough. I have to hard code a larger size in the get_subpartitions_size() function at pmb/install/install.py.

e.g.

full *= 1.20

# my hard coded value
full += 2200 * 1024 * 1024

boot += 15 * 1024 * 1024
return (full, boot)

It would be nice if this was a configuration option that was asked during /pmbootstrap.py init.