Set ZF prior to keyboard read call in check_for_keystroke().

Set the ZF flag to make sure the keyboard interrupt is actively
clearing it on a key event.  This fixes a hang when CONFIG_BOOTMENU is
on and CONFIG_KEYBOARD is off.

Signed-off-by: Kevin O'Connor <kevin@koconnor.net>
diff --git a/src/util.c b/src/util.c
index dcc0b71..ccf84b0 100644
--- a/src/util.c
+++ b/src/util.c
@@ -260,7 +260,7 @@
 {
     struct bregs br;
     memset(&br, 0, sizeof(br));
-    br.flags = F_IF;
+    br.flags = F_IF|F_ZF;
     br.ah = 1;
     call16_int(0x16, &br);
     return !(br.flags & F_ZF);