BuildSystem: Allow running qemu without kvm

If you set QEMU_ACCEL to empty string, build system won't try to look
for kvm support.

script/build.sh

@@ -68,7 +68,9 @@ run_bochs () {
 	$BANAN_SCRIPT_DIR/bochs.sh $@
 }
 
-if type kvm-ok &> /dev/null; then
+if [ -v QEMU_ACCEL ]; then
+	:
+elif type kvm-ok &> /dev/null; then
 	if kvm-ok &> /dev/null; then
 		QEMU_ACCEL="-accel kvm"
 	fi