Increase APT cache on demand

"elbe create example.xml" failed due to an out of room cache file.
Instead of simply increase the cache file set the startup cache size and
the cache grow. Also set the cache limit to unlimited.

$ elbe create example.xml
i386
Reading package lists... Done
Building dependency tree... Done
...
E: Dynamic MMap ran out of room. Please increase the size of APT::Cache-Start.
Current value: 32505856. (man 5 apt.conf)
Reading package lists... Error!
Failure to download kernel/initrd debian Package
Check your source URLs
$

Signed-off-by: Benedikt Spranger <b.spranger@linutronix.de>