[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/bootconfig.cpp

@@ -41,7 +41,8 @@ bootconfig::bootconfig(util::buffer data, uefi::boot_services *bs)
     data += 1; // reserved byte
     uint16_t num_programs = *util::read<uint16_t>(data);
     uint16_t num_data = *util::read<uint16_t>(data);
-    data += 2; // reserved short
+
+    m_flags = *util::read<uint16_t>(data);
 
     read_descriptor(m_kernel, data);
     read_descriptor(m_init, data);