MYNEWT-674 newt - Remove use of new io.SeekCurrent
MYNEWT-674 newt - Remove use of new io.SeekCurrent

This constant was added in go 1.7. A lot of package manager repos only
carry go 1.6, so the use of this constant makes newt unbuildable for a
lot of users.

apache/incubator-mynewt-newt,apache/incubator-mynewt-newt