Choose & set up a NTP client

From @ollieparanoid in https://github.com/postmarketOS/pmbootstrap/issues/570#issuecomment-329855915:

A few notes regarding ntpd:

• If we set the hardware clock to UTC/GMT instead of the localtime, we don't need to know the time zone beforehand (it will get calculated based on the UTC time then) (more info)
• Although that might not be obvious, we should take extra care choosing a suitable ntpd, especially when we want to have it running all the time. After all, it connects to the internet and executes code based on the input it gets, so it has a similar attack surface than curl or wget. Besides busybox' ntpd, there is openntpd from OpenBSD for example, which does proper privilege separation among other features. It is packaged for Alpine.
• Also running an ntpd creates characteristic traffic, which may be used to fingerprint your OS/device (I know that we'll need it for the N900, but we should keep that in mind at least).