The RTC-based entropy source uses the nanosecond-scale CPU TSC to
measure the time between two 1kHz interrupts generated by the CMOS
RTC.  In a physical machine these clocks are driven from independent
crystals, resulting in some observable clock drift.  In a virtual
machine, the CMOS RTC is typically emulated using host-OS
constructions such as SIGALRM.

Signed-off-by: Michael Brown <mcb30@ipxe.org>