boot: Fall back to using image load address for stack guard

If we cannot get random data from the RNG protocol we can still use our
load address to make the stack guard a little less static.

src/boot/efi/log.c

@@ -79,6 +79,9 @@ void __stack_chk_guard_init(void) {
         EFI_RNG_PROTOCOL *rng;
         if (BS->LocateProtocol(MAKE_GUID_PTR(EFI_RNG_PROTOCOL), NULL, (void **) &rng) == EFI_SUCCESS)
                 (void) rng->GetRNG(rng, NULL, sizeof(__stack_chk_guard), (void *) &__stack_chk_guard);
+        else
+                /* Better than no extra entropy. */
+                __stack_chk_guard ^= (intptr_t) &__ImageBase;
 }
 #endif