boot0cfg: Also allow a file as a trailing argument

[jlduran]

Summary

Modify the disk check to allow arbitrary files as the trailing argument
instead of requiring a live GEOM disk provider.

This enables modifying a boot0 binary file in-place before flashing it
to a disk via gpart bootcode, or using it directly as an argument to
mkimg's partition specification, as these tools cannot directly adjust
the parameters of the boot0 boot manager.

[jlduran]

I did this modification, mostly to allow using it in conjunction with mkimg (see the next review in the series). I cannot immediately see any downsides of also enabling a file for modification, I also didn't know whom to ask for a review, as boot0cfg's usage is slowly fading away.

[imp]

Ah! This explains the other issue I had in a review I saw before this.

[jlduran]

• Add more checks

[jhb]

LGTM in general, just some cosmetic suggestions.

usr.sbin/boot0cfg/boot0cfg.8

188

I wonder if we want to avoid modifying boot0 in place in examples, but instead suggest copying it to another pathname that is then passed to gpart bootcode? You could then do a combined example maybe:

Create a copy of
.Pa boot0
with only slices 1 and 3 enabled in the menu and install that onto a disk using
.Xr gpart 8 :
.Bd -literal -offset indent -compact
cp /boot/boot0 /boot/boot0.custom
boot0cfg -m 0x5 /boot/boot0.custom
gpart bootcode -b /boot/boot0.custom ada0
.Ed

usr.sbin/boot0cfg/boot0cfg.c

406

Pre-existing bug in the comment, but might as well fix it while you are here.

416

s/flash/write/?

Also, maybe "If name is a regular file, re-open it in write-only mode ..."