[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/libraries/bootproto/include/bootproto/kernel.h

@@ -114,8 +114,10 @@ struct frame_block
 
 enum class boot_flags : uint16_t {
     none  = 0x0000,
-    debug = 0x0001
+    debug = 0x0001,
+    test  = 0x0002,
 };
+is_bitfield(boot_flags);
 
 struct args
 {