eject: copy po subdirectory for out-of-tree build

The po subdirectory found on the VPATH for an out-of-tree build so we
copy it over. See [1] for discussion.

[1] http://lists.openembedded.org/pipermail/openembedded-devel/2014-May/096052.html



v3: only create and populate po/ if it doesn't already exist
v4: test to see if we're building out-of-tree

Signed-off-by: Ash Charles <ashcharles@gmail.com>
Reviewed-by: Otavio Salvador <otavio@ossystems.com.br>
Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>