build:refactor arm_sdimage.sh as to no longer require root.

There where two reasons for needing root access while making ARM builds. The
first reason was that we needed root permissions to create device nodes and
assign permissions to files. The second reason we needed root was that we used
loop mounted files to copy the build output to the file backed storage.

The first problem is solved by building in unprivileged mode (adding -U) during
compilation. The second problem is solved by using mcopy to put files on the
fat partition and use the mkfs.mfs proto files to create MFS file system.

Future work might be to replace this script by the netbsd makefs tool.

Change-Id: Ibba61d4cad053f7c1ede26169d5214065a4939d8

docs/UPDATING

@@ -1,3 +1,8 @@
+20130422:
+	The ARM build no longer requires you to build as root user you
+	therefore need to wipe (or chmod) your build generated files 
+	and start building using the -U option of build.sh. 
+
 20130312:
 	/bin/ksh was imported and needs to be added to your /etc/shells.
 	Run this command to add it: