Add mkimg to the list of bootstrap tools

Having this as part of the bootstrap tools is useful to build disk images
to boot in QEMU (especially when building on a Linux/macOS host where
mkimg is not available). We have been bootstrapping mkimg in CheriBSD for
a long time (using LOCAL_XTOOL_DIRS) but I believe this is also useful
upstream.

Reviewed By: emaste, brooks
Differential Revision: https://reviews.freebsd.org/D27602