stand: userboot_fmtdev can be reduced to devformat

devformat produces the same output as userboot_fmtdev, so just use it to
reduce on the dependencies. In addition, we don't need to use the
incomplete struct userboot_devdesc type, we can use struct devdesc
instead (in fact, there's no userboot_devdesc defined anywhere).

Sponsored by: Netflix
Reviewed by: jhb (prior version)
Differential Revision: https://reviews.freebsd.org/D35925

(cherry picked from commit 1e9b23448a3010e8403eae08ad890fe1eb75da8b)