ramips: Fix sysupgrade for Xiaomi mir3g

ramips: Fix sysupgrade for Xiaomi mir3g

Without this change sysupgrade from 18.06 to 19.07 is only possible with
the -F option.
In OpenWrt 18.06 the nand_do_platform_check() function is called with
the board name mir3g only, if the tar does not use mir3g it will fail.
OpenWrt 19.07 and later support the metadata with the supported_devices
attribute to allow renaming. Do the renaming of the target between 19.07
and master like it is done for some other boards.

I tested the following sysupgrades successfully without -F
18.06 -> 19.07
19.07 -> master
master -> 19.07

Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>