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/device/acpi.zig

@@ -16,6 +16,7 @@
 
 const std = @import("std");
 const danos = @import("danos");
+const config = @import("config");
 const device = @import("device.zig");
 const aml = @import("aml/aml.zig");
 const DeviceTree = device.DeviceTree;
@@ -116,7 +117,7 @@ pub const CpuInfo = struct {
     dropped: usize = 0,
 };
 
-const max_cpus = danos.max_cpus;
+const max_cpus = config.max_cpus;
 
 /// Filled in by `discover` (from the MADT); SMP bring-up reads it to wake the APs.
 pub var cpu_info: CpuInfo = .{};