ramips: Fix sysupgrade for Xiaomi mir3g

ramips: Fix sysupgrade for Xiaomi mir3g

This mostly reverts the original commit e9929ebeeaef ("ramips: Fix
sysupgrade for Xiaomi mir3g") and replaces it with setting the
BOARD_NAME to the old value.

This way the folder in the tar will be named sysupgrade-mir3g and not
sysupgrade-xiaomi_mir3g and the sysupgrade in OpenWrt 18.06 can find it.

Without this change sysupgrade from 18.06 to 19.07 is only possible with
the -F option.

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>