[9front] Update system: Boot files not updated

[Benjamin Riefenstahl <b.riefenstahl@turtle-trading.net>]

Another problem I encountered with "mk all; mk install" was that the
boot files were not re-created, the kernel, bootsectors and similar:

     --rw-rw-r-- M 24 sys sys    7960 Jul 18  2021 /386/9bootfat
     --rw-rw-r-- M 24 sys sys    6472 Jul 18  2021 /386/9boothyb
     --rw-rw-r-- M 24 sys sys    6496 Jul 18  2021 /386/9bootiso
     --rw-rw-r-- M 24 sys sys    8004 Jul 18  2021 /386/9bootpxe
     --rw-rw-r-- M 24 sys sys   11108 Jul 18  2021 /386/bootia32.efi
     --rw-rw-r-- M 24 sys sys   12356 Jul 18  2021 /386/bootx64.efi
     --rw-rw-r-- M 24 sys sys 1048576 Jul 18  2021 /386/efiboot.fat
     --rw-rw-r-- M 24 sys sys     407 Jul 18  2021 /386/mbr
     --rw-rw-r-- M 24 sys sys     473 Jul 18  2021 /386/pbs
     --rwxrwxr-x M 24 sys sys 5229573 Jul 18  2021 /amd64/9pc64

Is this expected?

To get these compiled and installed, I now do something like:

        for (d in 9/pc64 boot/efi boot/pc) @{
            cd $d
            mk install
        }

Even that procedure does not re-install them into the 9fat partition.
Is there a ready-made script or mkfile target for that?

Thanks, benny