Make busybox 1.20.2 default

Fixes #47

1.20.0 has a nasty bug, which is fixed in 1.20.2:

https://bugs.busybox.net/show_bug.cgi?id=5282

  Result of variable substitution in right hand side of I/O
  redirection causes trailing garbage byte.

Signed-off-by: Curt Brune <curt@cumulusnetworks.com>