common/uboot: Explicitly separate kernel/user arguments

This way, we avoid arguments intended for userspace to be accidentally
parsed by the kernel.

board/aarch64/rootfs/boot/syslinux/syslinux.conf

@@ -1,4 +1,4 @@
 label Infix (aarch64)
 	kernel /boot/Image
 	fdtdir /boot
-	append ${bootargs_root} ${bootargs_rauc} ${bootargs_log}
+	append ${bootargs_root} ${bootargs_log} -- ${bootargs_user}

board/common/uboot/scripts/ixbootslot.sh

@@ -14,7 +14,7 @@ if test "${prepared}" = "ok"; then
 	if iminfo ${ramdisk_addr_r}; then
 	    echo "${slot}: Booting..."
 
-	    setenv bootargs_rauc "rauc.slot=${slot}"
+	    setenv bootargs_user "rauc.slot=${slot}"
 
 	    blkmap create boot
 	    blkmap get boot dev blkmapnum