Re: amd64-current live image build failure

[Izumi Tsutsui <tsutsui%ceres.dti.ne.jp@localhost>]

> cat  work.mbr.truncated work.efi imgroot.fs work.gpt > work.img
> /dumps/sysbuild/amd64/tools/bin/nbgpt  work.img biosboot -i 2
>   -c /dumps/sysbuild/amd64/obj/home/sysbuild/src/distrib/amd64/liveimage/emuimage/work/usr/mdec/gptmbr.bin
> nbgpt: work.img: No primary GPT header; run create or recover

If you are trying an update build (with -u) and your previous build
was using source tree files before UEFI changes against liveimage,
 https://mail-index.netbsd.org/source-changes/2024/04/13/msg150802.html
a stale work.mbr (${WORKMBR}) image doesn't contain GPT (but MBR)
so ${TOOL_GPT} on the update build may fail.

The similar issue (against the secondary GPT headers) occured
if IMAGE size is changed:
 https://gnats.netbsd.org/56132

Maybe we should note it (remove work.mbr on updating builds)
in doc/UPDATING?

---
Izumi Tsutsui