sysutils/fwup: new port for managing embedded linux systems

fwup is a configurable image-based software update utility for embedded
Linux-based systems. It primarily supports software upgrade strategies
that update entire root filesystem images at once. This includes
strategies like swapping back and forth between A and B partitions,
recovery partitions, and various trial update/failback scenarios. All
software update information is combined into a ZIP archive that may
optionally be cryptographically signed. fwup has minimal dependencies
and runtime requirements.

WWW: https://github.com/fhunleth/fwup

Approved by: jrm (mentor)
Differential Revision: https://reviews.freebsd.org/D17907