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>

ANS X9.82 specifies several Approved Sources of Entropy Input (SEI).
One such SEI uses an entropy source as the Source of Entropy Input,
condensing each entropy source output after each GetEntropy call.
This can be implemented relatively cheaply in iPXE and avoids the need
to allocate potentially very large buffers.

(Note that the terms "entropy source" and "Source of Entropy Input"
are not synonyms within the context of ANS X9.82.)

Use the iPXE API mechanism to allow entropy sources to be selected at
compilation time.

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