install/54906: Sysinst always says "the bootcode in the Master Boot Record does not appear to be valid"

[guest01%cyberia.pl@localhost]

>Number:         54906
>Category:       install
>Synopsis:       Sysinst always says "the bootcode in the Master Boot Record does not appear to be valid"
>Confidential:   no
>Severity:       serious
>Priority:       medium
>Responsible:    install-manager
>State:          open
>Class:          sw-bug
>Submitter-Id:   net
>Arrival-Date:   Tue Jan 28 22:00:01 +0000 2020
>Originator:     guest01
>Release:        8.1
>Organization:
-
>Environment:
netBSD localhost 8.1_STABLE NetBSD 8.1_STABLE (GENERIC) #0: Tue Jan  7 12:01:32 UTC 2020  mkrepro%mkrepro.NetBSD.org@localhost:/usr/src/sys/arch/amd64/compile/GENERIC amd64

>Description:
When you install system, sysinst ask:
"The bootcode in the Master Boot Record does not appear to be valid. Do you want to install the NetBSD bootcode ?"
You answer "Yes" and you go to the end of the installation.
You have system installed.
Then you repeat installation on the same computer and sysinst ask again:
"The bootcode in the Master Boot Record does not appear to be valid. Do you want to install the NetBSD bootcode ?"

When bootcode will be valid ?
>How-To-Repeat:
1. Install system without changing MBR partitions and NetBSD partitions (except newfs) selecting yes to install the NetBSD bootcode.
2. Install system second time without changing MBR partitions and NetBSD partitions
>Fix: