Create a large enough seedfile

The seedfile needs to have the size of the entropy accumulator, which
is 64 bytes (512 bits) since the entropy accumulator uses SHA-512 and
the seed size needs to be the same as the hash output (or larger).

We used to enable MBEDTLS_ENTROPY_FORCE_SHA256 in the full config, so
the entropy accumulator was 256 bits (32 bytes), and therefore a
32-byte seedfile worked. But we no longer turn on this option in the
full config, so the 32-byte seedfile no longer works.

Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>

tests/scripts/basic-build-test.sh

@@ -76,7 +76,7 @@ make -j
 TEST_OUTPUT=out_${PPID}
 cd tests
 if [ ! -f "seedfile" ]; then
-    dd if=/dev/urandom of="seedfile" bs=32 count=1
+    dd if=/dev/urandom of="seedfile" bs=64 count=1
 fi
 
 # Step 2a - Unit Tests (keep going even if some tests fail)