allow additional busybox applets to be build from commandline

expected commandline:
```
./mkroot.sh BUSYBOX_RPM=y
```

expected contents of mini.conf:
```
CONFIG_RPM=y
```


hmm ... since cmd options like this are already propagated into env, after writing basic mini.conf one could do:
```
env | grep ^BUSYBOX_ | sed -e 's|^BUSYBOX_|CONFIG_|g' >> mini.conf
```