Add /boot/overlays to runtime pkg, fix distrib-dirs METALOG generation

/boot/overlays was recently added without belonging to a package. It's only
used by bootloaders at the moment, so add it to the 'runtime' package to get
added with ubldr and friends.

Fix distrib-dirs METALOG generation while we're here. History elsewhere
seems to indicate that bapt@ fixed this to pull in all attributes from
mtrees while generating the METALOG. This fix got clobbered somewhere later,
so restore it.

Reviewed by: bapt, gjb
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D13996