kernel: enable ramfs build and automount to shared_memory directory

This will allow proper operation of POSIX SHM API (shm_open etc.).
Now memory files are stored fully in memory and do not affect disk
storage (except swap if enabled).

Change-Id: Iae3ce1afa968df72e82198e598a273cbf7cb0269
Reviewed-on: https://review.haiku-os.org/c/haiku/+/5802
Reviewed-by: Fredrik Holmqvist <fredrik.holmqvist@gmail.com>
Reviewed-by: waddlesplash <waddlesplash@gmail.com>
Tested-by: Commit checker robot <no-reply+buildbot@haiku-os.org>
(cherry picked from commit bf2786efae)
Reviewed-on: https://review.haiku-os.org/c/haiku/+/5851

build/jam/images/definitions/minimum

@@ -243,6 +243,7 @@ SYSTEM_ADD_ONS_FILE_SYSTEMS =
 	log_overlay
 	ntfs
 	packagefs
+	ramfs
 	reiserfs
 	udf
 	write_overlay

src/system/kernel/fs/vfs_boot.cpp

@@ -555,6 +555,10 @@ vfs_mount_boot_file_system(kernel_args* args)
 		}
 	}
 
+	status = _kern_mount("/boot/system/var/shared_memory", NULL, "ramfs", 0, NULL, 0);
+	if (status < B_OK)
+		dprintf("Failed to mount shared memory FS: %s\n", strerror(status));
+
 	// Now that packagefs is mounted, the boot volume is really ready.
 	gBootDevice = bootDevice;