butch template: use "." instead of "source"

closes #238

"source" is a bashism, although one that makes sense (and is thus supported
by busybox ash), because "." is very hard to spot and to differentiate
from "./".
usage of the source keyword was already hunted down and removed a while ago,
( 7bb3fcc60ac656d34ed511972df579d4ba7a1a88 )
but later reintroduced into the butch template.