[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.

src/kernel/syscalls/system.cpp

@@ -36,6 +36,14 @@ noop()
     return j6_status_ok;
 }
 
+[[ noreturn ]] j6_status_t
+test_finish(uint32_t exit_code)
+{
+    // Tell QEMU to exit
+    asm ( "out %0, %1" :: "a"(exit_code), "Nd"(0xf4) );
+    while (1) asm ("hlt");
+}
+
 j6_status_t
 system_get_log(system *self, void *buffer, size_t *buffer_len)
 {