Improve RTC handling

[kpfleming]

• Add 'abx80x' to list of possible RTC drivers

• Add 'module_enable' function in installer script for modules to be loaded in installed system

• Use 'dtoverlay_enable' function in various places it was not being used

• Ensure module for the RTC has been loaded during installation

• Stop adding 'hwclock' call to /etc/rc.local as it is no longer needed

  • systems using sysvinit already have a udev rule to do this
  • systems using systemd will use service unit described below

• Add systemd service unit to read RTC on startup

  • as seen in Debian bug 855203, systems using systemd don't read the RTC during startup if the RTC's driver is loaded as a module; since all Raspberry Pi kernels are configured this way, a service unit is required to read the RTC during system startup

• Ensure module for the RTC will be loaded in installed system

• Ensure that RTC date/time is set in UTC mode

The 'ensure module loaded' bits are required because at least one RTC module (abx80x) is not automatically loaded even though the proper Device Tree overlay has been configured. For those which are autoloaded, having the module name in /etc/modules (or /etc/modules-load.d/rtc.conf) won't cause any harm.

Signed-off-by: Kevin P. Fleming kevin@km6g.us

[kpfleming]

This has some conflicts with #186; I'll fix it up after that is merged.