vbutil_what_keys: support 16 MB firmware images

Previously, vbutil_what_keys assumed any input files
of size 8 MB or lower are firmware images.  Push that
size up to 16 MB to support larger firmware images,
such as that of eve's.

BUG=None
TEST=vbutil_what_keys on eve image.bin

Change-Id: Iaf07ad3f419f5e79584391a2b846100e3fae61dc
Reviewed-on: https://chromium-review.googlesource.com/1156326
Commit-Ready: Joel Kitching <kitching@chromium.org>
Tested-by: Joel Kitching <kitching@chromium.org>
Reviewed-by: Bill Richardson <wfrichar@chromium.org>

utility/vbutil_what_keys

@@ -98,7 +98,7 @@ dofile() {
   file="$1"
   size=$(stat -c %s "$file")
 
-  if [ -f "$file" ] && [ "$size" -le 8388608 ]; then
+  if [ -f "$file" ] && [ "$size" -le 16777216 ]; then
     echo -e "\nBIOS: $file"
 
     showbios "$file" ""