typekit: correct the internal include path to Types.hpp

Don't assume yet that ../.. is in the include path when compiling Convertions.hpp...
whish is the case when we use these functions directly from an app.

Signed-off-by: Peter Soetens <peter@thesourceworks.com>
typegen: don't overwrite the OROCOS_TARGET if already set.

Signed-off-by: Peter Soetens <peter@thesourceworks.com>