For some reason, during system the main loop exits before we call
g_main_loop_quit(), so don't rely on it and use a simple polling loop to
check for modem poweroff.
`Developer edition` phones have a different compatible string but should
be considered identical to BraveHeart edition here. Therefore BH is the
default phone, unless the compatible is that of a CE phone.
When powering off the modem, we must assert the RESET line only when the
modem has shut down (otherwise it'll cause a hard reset and won't allow
the modem to shut down properly).
This commit also polls the STATUS pin during poweroff and quits
immediately once this pin goes high.
