ramips: add dwr-512 jboot firmware configuration

The previous fw version require the replacement of the stock bootloader
with u-boot. This prevent an easy stock restore of the original fw.

Now a proper fw util has been developed to manage the stock jboot
bootloader. Therefore make sense have a fw image for the stock
bootloader.

The old fw configuration (u-boot) is not compatible with the new one
and will not be supported anymore.

So at the end 2 image can be generated:

1) factory image with jboot bootloader
     openwrt-ramips-rt305x-dwr-512-b-squashfs-factory.bin
2) sysupgrade image with jboot bootloader
     openwrt-ramips-rt305x-dwr-512-b-squashfs-sysupgrade.bin

Signed-off-by: Giuseppe Lippolis <giu.lippolis@gmail.com>

target/linux/ramips/base-files/lib/upgrade/platform.sh

@@ -52,7 +52,6 @@ platform_check_image() {
 	dir-620-d1|\
 	dir-810l|\
 	duzun-dm06|\
-	dwr-512-b|\
 	e1700|\
 	esr-9753|\
 	ew1200|\
@@ -272,7 +271,8 @@ platform_check_image() {
 		return 0
 		;;
 	dlink,dwr-116-a1|\
-	dlink,dwr-921-c1)
+	dlink,dwr-921-c1|\
+	dwr-512-b)
 		[ "$magic" != "0404242b" ] && {
 			echo "Invalid image type."
 			return 1