[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/boot/main.cpp

@@ -77,6 +77,7 @@ load_resources(bootproto::args *args, video::screen *screen, uefi::handle image,
 
     args->kernel = loader::load_program(disk, bc.kernel(), true);
     args->init = loader::load_program(disk, bc.init());
+    args->flags = static_cast<bootproto::boot_flags>(bc.flags());
 
     namespace bits = util::bits;
     using bootproto::desc_flags;