[kernel] Add test mode, controlled by manifest

The manifest can now supply a list of boot flags, including "test".
Those get turned into the bootproto::args::flags field by the
bootloader. The kernel takes those and uses the test flag to control
enabling syscalls with the new "test" attribute, like the new
test_finish syscall, which lets automated tests call back to the kernel
to shut down the system.

assets/manifests/test.yaml

@@ -0,0 +1,8 @@
+---
+init: srv.init
+flags: ["test"]
+programs:
+  - name: panic.serial
+    target: kernel
+    flags: panic
+  - name: drv.uart