gather kernel tunables into config.zig

max_cpus, max_tasks, kernel/IST stack sizes, and timer_hz move from scattered constants into one config module. root.zig goes back to being just the boot contract. Values unchanged; any can become a -D build option later.

src/kernel/scheduler.zig

@@ -18,7 +18,7 @@
 //! shared queues.
 
 const std = @import("std");
-const danos = @import("danos");
+const config = @import("config");
 const arch = @import("arch");
 const heap = @import("heap.zig");
 const sync = @import("sync.zig");
@@ -27,8 +27,8 @@ const sync = @import("sync.zig");
 pub const Priority = u3;
 const num_priorities = 8;
 
-const stack_size = 16 * 1024; // each task's kernel stack is 16 KiB
-const max_tasks = 16; // the maximum number of tasks alive at once is 16 in a static sized pool
+const stack_size = config.kernel_stack_size; // each task's kernel stack
+const max_tasks = config.max_tasks; // maximum tasks alive at once (static pool)
 
 const State = enum { free, ready, running, blocked };
 
@@ -69,7 +69,7 @@ pub const PerCpu = struct {
     pinned_bitmap: u8 = 0,
 };
 
-const max_cpus = danos.max_cpus;
+const max_cpus = config.max_cpus;
 var cpus = [_]PerCpu{.{}} ** max_cpus;
 
 /// This core's per-CPU state, via the arch layer's GS-base pointer. Valid only once