This change allows one to set the busy_detect flag
required by the synopsys UART at the loader prompt.
This is needed by the EPYC 3000 SoC.

This will give users a working console up to the point where getty is required:
hw.uart.console="mm:0xfedc9000,rs:2,bd:1"

Reviewed by:	imp
MFC after:	4 weeks
Differential Revision:	https://reviews.freebsd.org/D16399