allow additional busybox applets to be build from commandline

[terefang]

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