vmbase: Move rand.rs out of pvmfw

The module isn't pvmfw-specific and could be re-used by other
vmbase-based code so move it. This will allow better future integration
with the TRNG already performed in vmbase/entry.S and with the C
interface exported by vmbase/src/bionic.rs.

Test: TH
Change-Id: I842425264c87cfe97618aceda371b410d7af6046
diff --git a/pvmfw/src/entry.rs b/pvmfw/src/entry.rs
index 0d2dfda..e2c5fc9 100644
--- a/pvmfw/src/entry.rs
+++ b/pvmfw/src/entry.rs
@@ -19,7 +19,6 @@
 use crate::fdt;
 use crate::heap;
 use crate::memory;
-use crate::rand;
 use core::arch::asm;
 use core::mem::{drop, size_of};
 use core::num::NonZeroUsize;
@@ -38,6 +37,7 @@
     logger, main,
     memory::{min_dcache_line_size, MemoryTracker, MEMORY, SIZE_4KB},
     power::reboot,
+    rand,
 };
 use zeroize::Zeroize;