mdadm: Fix config generation

The init script generated something like "DEVICE=/dev/sda" when it should
have been generating "DEVICE /dev/sda". mdadm errors on this. Patch by jow.

Also changed the default sendmail path to /usr/sbin/sendmail. No package
in LEDE provides /sbin/sendmail. msmtp provides /usr/sbin/sendmail so use
that.

Also add a patch to fix file paths for mdadm runtime files. mdadm currently
errors on them since /run is missing. Once /run is added to stock LEDE, this
patch can be removed.

Signed-off-by: Rosen Penev <rosenp@gmail.com>
[rewrap commit message]
Signed-off-by: Jo-Philipp Wich <jo@mein.io>

package/utils/mdadm/files/mdadm.init

@@ -34,14 +34,13 @@ mdadm_common() {
 	local cfg="$1"
 	local email devices
 
-	if [ -x /sbin/sendmail ]; then
-		append_option email "$cfg" email "MAILADDR"
-
+	if [ -x /usr/sbin/sendmail ]; then
+		config_get email "$cfg" email
+		[ -n "$email" ] && printf "MAILADDR %s\n" "$email" >> $CONF
 	fi
 
-	append_option devices "$cfg" devices DEVICE " "
-
-	printf "%s\n%s\n" "$email" "$devices" >> $CONF
+	config_list_foreach "$cfg" devices append_list_item devices " "
+	[ -n "$devices" ] && printf "DEVICE %s\n" "$devices" >> $CONF
 }
 
 mdadm_array() {