luci-mod-network: split config migration into 2 steps

Problem with handling all migrations in 1 step is that uci.sections()
doesn't include changes queued using uci.callAdd() and uci.callSet().
That could result in unexpected behaviour and generating invalid
configs.

For the sake of simplicity and reliability use 2 steps migration. The
downside is that users may get prompted twice to migrate.

Reported-by: Hauke Mehrtens <hauke@hauke-m.de>
Fixes: 74be304e

 ("treewide: use "device" option in UCI "interface" sections")
Signed-off-by: Rafał Miłecki <rafal@milecki.pl>
Tested-by: Hauke Mehrtens <hauke@hauke-m.de>