- move "MD instructions on setting up boot media"
from common/xfer with messy mdoc .if conditionals
into MD xfer files so that developers can see
what contents are missing or obsolete and should be
written or updated
- remove some useless MD description for maintainability

XXX: still a bunch of unmaintained .if \n [${MACHINE}] in common files

branches: 1.1.2; 1.1.4;
First cut at INSTALL notes for NetBSD/arc.
Finally we could close another ancient PR port-arc/11443.

After several years, we got the native bootloader and then had
sysinst which would do apprpriate partitioning for the bootloader,
so I've noted the last difficult part, how to specify boot commands
on the ARC BIOS.

I still wonder how many possible users of NetBSD/arc are there
who will need this topic though... ;-p