RPi4 has a different root device, so use config.txt conditionals to load

an alternate cmdline txt file (cmdline-pi4.txt) on these boards.

distrib/utils/embedded/conf/arm64.conf

@@ -1,4 +1,4 @@
-# $NetBSD: arm64.conf,v 1.8 2019/12/16 11:00:30 skrll Exp $
+# $NetBSD: arm64.conf,v 1.9 2020/02/23 10:56:03 jmcneill Exp $
 # ARM64 customization script used by mkimage
 #
 board=arm64
@@ -71,6 +71,10 @@ populate_rpi() {
 
 	cat > "${mnt}/boot/cmdline.txt" << EOF
 root=ld0a console=${console}
+EOF
+
+	cat > "${mnt}/boot/cmdline-pi4.txt" << EOF
+root=ld1a console=${console}
 EOF
 
 	cat > "${mnt}/boot/config.txt" << EOF
@@ -82,6 +86,11 @@ kernel=netbsd.img
 kernel_address=0x200000
 enable_uart=1
 force_turbo=0
+#
+[pi4]
+cmdline=cmdline-pi4.txt
+#
+[all]
 EOF
 
 	echo "${bar} installing firmware files ${bar}"