some significant differences:
* Because the pmax bootblocks don't have a fixed load address, we
need to keep the first stage blocks in /usr/mdec with some sort
of executable format that includes load and transfer address.
Since ELF is native, /usr/mdec/bootxx_* are in ELF format and
installboot (in loadbootstrap.c) needs knowledge of the ELF format.
* Support for installing the first stage at an arbitiary position.
-a adds the first stage to end of the "disk" (only on works on
regular files) and -i <block> puts the first stage at ISO
filesystem block number <block> for use in multi-arch boot
CDs (see installboot(8) for more details and an example).
* No checksum support. This should really be done in some sort of
MI way rather than duplicating code between ports' installboot.c
