[rng] Add ANS X9.82 Approved Source of Entropy Input

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>

src/config/defaults/efi.h

@@ -17,6 +17,7 @@
 #define SMBIOS_EFI
 #define SANBOOT_NULL
 #define BOFM_EFI
+#define ENTROPY_NULL
 
 #define	IMAGE_EFI		/* EFI image support */
 #define	IMAGE_SCRIPT		/* iPXE script image support */