Screen modes in fastboot file

We still have lines in the fastboot file to change screen mode before
booting NetBSD.

Currently, we have:

| Select the monitor defintion file and screenmode
Set NetBSD$MDF ADFS::4.$.AKF85
Set NetBSD$ScreenMode "X1024 Y768 C256"

[...]

loadmodefile <NetBSD$MDF>
Set Alias$SetNetBSDMode Wimpmode <NetBSD$ScreenMode>
SetNetBSDMode


This was left in after previous discussion after fears were aired that the
fastboot mechanism (by this I refer to starting NetBSD without booting
into the desktop) will activate before RISC OS loads the monitor
definition file and sets the screenmode. However, this leaves the problem
that after installing !BtNetBSD, until the fastboot file is edited,
booting will fail with a complaint about the file ADFS::4.$.AKF85 not
being found. Yet further configuration needs to be done to boot on a
monitor that doesn't support the chosen screen mode. This is inconvenient
to say the least.

I have tested the bootloader with all of these lines commented out, both
from the desktop and from fastboot, and both cases work correctly, using
the screen mode configured in RISC OS.

In the light of this, I intend to change the MDF path to one that is
likely to exist in RISC OS, comment out the Set NetBSD$... lines and make
the last lines conditional on these variables being set. I also intend to
add a comment saying that NetBSD will normally boot using the screen mode
configured in RISC OS, but to uncomment and edit one or both of the two
lines below to boot NetBSD in a different screen mode to RISC OS.

If there are no objections, I intend to commit later this week.

-- 
Gillette - the best a man can forget