[MDC-461] simetbox-openwrt-simet-ma: contact controller at boot

[MDC-461] simetbox-openwrt-simet-ma: contact controller at boot

Attempt to download a new schedule at boot, if succesfully registered.
This is done assyncronously, so we will likely run for a while with the
default schedule.

Don't clobber the downloaded schedule if we race a schedule download.