stand: i386_fmtdev can be reduced to devformat

devformat produces the same output as i386_fmtdev, so just use it to
reduce on the dependencies.

Sponsored by: Netflix
Reviewed by: tsoome
Differential Revision: https://reviews.freebsd.org/D35927

(cherry picked from commit 362322294045943703e03dee263bbc237002525c)