[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/cpu.cpp

@@ -98,8 +98,6 @@ bsp_late_init()
     asm ("mov %%cr4, %0" : "=r"(cr4));
     uint64_t efer = rdmsr(msr::ia32_efer);
     log::debug(logs::boot, "Control regs: cr0:%lx cr4:%lx efer:%lx", cr0, cr4, efer);
-
-    syscall_initialize();
 }
 
 cpu_data *