fix(sys-kernel/bootengine): Add all dependencies required by postinst

When building on a fresh board root it is easy for bootengine to die a
silent but horrible death because it was installed before one of the
tools it uses to create or includes in the initird. To avoid this use a
very complete dependency list for both DEPEND and RDEPEND.
fix(sys-kernel/bootengine): Use new update-bootengine script in postinst

All the nasties have been polished and hidden away in a new script.
Merge pull request #114 from marineam/update-bootengine

Update bootengine