ramips: add dwr-512 jboot firmware configuration

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>