sculpt: improve USB hotplug handling

- Don't reset sculpt partition on USB changes whenever ram fs is
  selected for use.

- Reconfigure runtime immediately when plugging USB devices.
  This prevents the usb-block driver and part_block from
  dangling in the graph. They disappear immediately now.

repos/gems/src/app/sculpt_manager/model/usb_storage_device.h

@@ -147,6 +147,8 @@ struct Sculpt::Usb_storage_device_update_policy
 	Env       &_env;
 	Allocator &_alloc;
 
+	bool device_added_or_vanished = false;
+
 	Signal_context_capability _sigh;
 
 	Usb_storage_device_update_policy(Env &env, Allocator &alloc,
@@ -157,10 +159,17 @@ struct Sculpt::Usb_storage_device_update_policy
 
 	typedef Usb_storage_device::Label Label;
 
-	void destroy_element(Usb_storage_device &elem) { destroy(_alloc, &elem); }
+	void destroy_element(Usb_storage_device &elem)
+	{
+		device_added_or_vanished = true;
+
+		destroy(_alloc, &elem);
+	}
 
 	Usb_storage_device &create_element(Xml_node node)
 	{
+		device_added_or_vanished = true;
+
 		return *new (_alloc)
 			Usb_storage_device(_env, _alloc, _sigh,
 			                   node.attribute_value("label_suffix", Label()));

repos/gems/src/app/sculpt_manager/storage.cc

@@ -41,6 +41,9 @@ void Sculpt::Storage::handle_storage_devices_update()
 
 		_storage_devices.update_usb_storage_devices_from_xml(policy, raw);
 
+		if (policy.device_added_or_vanished)
+			reconfigure_runtime = true;
+
 		_storage_devices.usb_storage_devices.for_each([&] (Usb_storage_device &dev) {
 
 			dev.process_driver_report();
@@ -66,7 +69,12 @@ void Sculpt::Storage::handle_storage_devices_update()
 	 * target to use.
 	 */
 	if (_sculpt_partition.valid()) {
+
 		bool sculpt_partition_exists = false;
+
+		if (_sculpt_partition.ram_fs())
+			sculpt_partition_exists = true;
+
 		_storage_devices.for_each([&] (Storage_device const &device) {
 			device.for_each_partition([&] (Partition const &partition) {
 				if (device.label == _sculpt_partition.device