stand: Add /boot/overlays to allow separation of overlays from base FDT

This matches directory structure used commonly in Linux-land, and it's
cleaner than mixing overlays into the existing module paths. Overlays are
still mixed in by specifying fdt_overlays in loader.conf(5).

Reviewed by:	manu
Differential Revision:	https://reviews.freebsd.org/D13922

etc/mtree/BSD.root.dist

@@ -18,6 +18,8 @@
         ..
         modules
         ..
+        overlays
+        ..
         zfs
         ..
     ..

share/man/man7/hier.7

@@ -28,7 +28,7 @@
 .\"	@(#)hier.7	8.1 (Berkeley) 6/5/93
 .\" $FreeBSD$
 .\"
-.Dd December 3, 2017
+.Dd January 15, 2018
 .Dt HIER 7
 .Os
 .Sh NAME
@@ -63,6 +63,11 @@ at boot time) and kernel modules
 third-party loadable kernel modules;
 see
 .Xr kldstat 8
+.It Pa overlays/
+Compiled flattened device tree (FDT) overlays; see
+.Xr fdt 4
+and
+.Xr dtc 1
 .It Pa zfs/
 .Xr zfs 8
 zpool cache files

stand/arm/loader/loader.conf

@@ -7,7 +7,7 @@ autoboot_delay=10
 bootfile="kernel"      # Kernel name (possibly absolute path)
 kernel="kernel"        # /boot sub-directory containing kernel and modules
 loader_conf_files="/boot/loader.conf /boot/loader.conf.local"
-module_path="/boot/kernel;/boot/modules;/boot/dtb"
+module_path="/boot/kernel;/boot/modules;/boot/dtb;/boot/overlays"
 nextboot_conf="/boot/nextboot.conf"
 nextboot_enable="NO"
 verbose_loading="NO"

stand/forth/loader.conf

@@ -97,7 +97,7 @@ 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"	# Set the module search path
+module_path="/boot/modules;/boot/dtb;/boot/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

stand/mips/uboot/loader.conf

@@ -7,7 +7,7 @@ autoboot_delay=10
 bootfile="kernel"      # Kernel name (possibly absolute path)
 kernel="kernel"        # /boot sub-directory containing kernel and modules
 loader_conf_files="/boot/loader.conf /boot/loader.conf.local"
-module_path="/boot/kernel;/boot/modules;/boot/dtb"
+module_path="/boot/kernel;/boot/modules;/boot/dtb;/boot/overlays"
 nextboot_conf="/boot/nextboot.conf"
 nextboot_enable="NO"
 verbose_loading="NO"