Makefile.uk: Improve generating the python rootfs

Makefile.uk: Improve generating the python rootfs

This change reworks the make targets that generate the python rootfs:
- Do not use hardcoded python version
- Provide the option of generating .pyc files in the root or not
- Error out or warn the user if a python version mismatch might subtly
  break the resulting rootfs

Signed-off-by: Andrei Tatar <andrei@unikraft.io>
Reviewed-by: Maria Sfiraiala <maria.sfiraiala@gmail.com>
Reviewed-by: Stefan Jumarea <stefanjumarea02@gmail.com>
Approved-by: Razvan Deaconescu <razvand@unikraft.io>
Tested-by: Unikraft CI <monkey@unikraft.io>
GitHub-Closes: #15