Simplify MBR and install it as /usr/mdec/mbr instead of /usr/mdec/mbr.mini

since it makes a fairly decent template for fdisk -i.  Change the procedure
that makes the rb600 installation miniroot image to use fdisk -i instead of
dd(1) to create the MBR for the image.  Adjust the miniroot disk type in
/etc/disktab to the slightly different layout that fdisk -i creates.

src/etc/etc.socppc/disktab

@@ -1,4 +1,4 @@
-# $OpenBSD: disktab,v 1.2 2010/02/15 13:46:50 kettenis Exp $
+# $OpenBSD: disktab,v 1.3 2010/02/15 22:56:21 kettenis Exp $
 #
 #	@(#)disktab	5.5 (Berkeley) 2/27/91
 #
@@ -24,5 +24,5 @@ rdroot|ramdiskroot|RAM-disk root FS image:\
 
 miniroot|Installation root FS image:\
 	:ty=simulated:se#512:nc#8:nt#16:ns#63:\
-	:ta=4.2BSD:oa#575:pa#7489:fa#512:ba#4096:\
+	:ta=4.2BSD:oa#1008:pa#7056:fa#512:ba#4096:\
 	:ob#0:pb#0:oc#0:pc#8064:ti=boot:oi#63:pi#512: