Move /boot/overlays to /boot/dtb/overlays

The former is fairly vague; these are FDT overlays to be applied to the
running system, so /boot/dtb is a sensible location to put it without
cluttering up /boot/dtb even further if desired.

etc/mtree/BSD.root.dist

@@ -11,6 +11,8 @@
         defaults
         ..
         dtb
+            overlays              tags=package=runtime
+            ..
         ..
         firmware
         ..
@@ -20,8 +22,6 @@
         ..
         modules
         ..
-        overlays              tags=package=runtime
-        ..
         zfs
         ..
     ..

stand/defaults/loader.conf

@@ -80,7 +80,7 @@ bootenv_autolist="YES"		# Auto populate the list of ZFS Boot Environments
 #comconsole_speed="9600"	# Set the current serial console speed
 #console="vidconsole"		# A comma separated list of console(s)
 #currdev="disk1s1a"		# Set the current device
-module_path="/boot/modules;/boot/dtb;/boot/overlays"	# Set the module search path
+module_path="/boot/modules;/boot/dtb;/boot/dtb/overlays"	# Set the module search path
 #prompt="\\${interpret}"	# Set the command prompt
 #root_disk_unit="0"		# Force the root disk unit number
 #rootdev="disk1s1a"		# Set the root filesystem