misc/56468: Boot countdown display glitch

[kim%netbsd.org@localhost]

>Number:         56468
>Category:       misc
>Synopsis:       Boot countdown display glitch
>Confidential:   no
>Severity:       non-critical
>Priority:       low
>Responsible:    misc-bug-people
>State:          open
>Class:          sw-bug
>Submitter-Id:   net
>Arrival-Date:   Wed Oct 27 13:20:00 +0000 2021
>Originator:     kim%netbsd.org@localhost
>Release:        NetBSD 9.99.92 (202110262340Z)
>Organization:
>Environment:
System: NetBSD rendez-vous.gw.fi 9.99.92 NetBSD 9.99.92 (GENERIC) #0: Tue Oct 26 23:37:40 UTC 2021 mkrepro%mkrepro.NetBSD.org@localhost:/usr/src/sys/arch/amd64/compile/GENERIC amd64
Architecture: x86_64
Machine: amd64
>Description:

	  \\-__,------,___.
	   \\        __,---`  NetBSD/x86 EFI Boot (x64)
	    \\       `---,_.  Revision 1.1 (Tue Oct 26 23:37:40 UTC 2021)
	     \\-,_____,.---`
	      \\
	       \\
		\\

	The EFI bootblock countdown is incorrectly appended to the
	output if you press enter:

	    booting netbsd - starting in 3 seconds. 0 seconds.

	The expected output would be to overwrite the number of
	remaining seconds with the zero. The code in would appear to
	be printing \b characters, but I don't know why that is not
	reflected on-screen.

	https://nxr.netbsd.org/xref/src/sys/stand/efiboot/prompt.c#115

	Interestingly enough this output is not visible on the serial
	console after booting. I guess the line is overwritten by the
	"commands(s):" output, such as:

	    command(s): consdev com0,0;rndseed /etc/entropy-file;boot

	Running on qemu q35.

>How-To-Repeat:

	Start or reboot the system.

>Fix: