add option to build stage5 without xbuild

3 jobs for stage5 in 13 seconds (queued for 2 seconds)
Status Job ID Name Coverage
  Prepare
passed #562032
linux shell
docker

00:00:03

 
  Build
failed #562033
docker
build

00:00:10

 
  Test
skipped #562034
docker
test
 
Name Stage Failure
failed
build Build
Checking out 32cbda25 as stage5...
Removing target/

Updating/initializing submodules...
$ make
/bin/sh: 1: bootimage: not found
make: *** [qemu] Error 1
Makefile:28: recipe for target 'qemu' failed
ERROR: Job failed: exit code 1