zfs label bootenv should store data as nvlist

nvlist does allow us to support different data types and systems.

To encapsulate user data to/from nvlist, the libzfsbootenv library is
provided.

Reviewed-by: Arvind Sankar <nivedita@alum.mit.edu>
Reviewed-by: Allan Jude <allan@klarasystems.com>
Reviewed-by: Paul Dagnelie <pcd@delphix.com>
Reviewed-by: Igor Kozhukhov <igor@dilos.org>
Signed-off-by: Toomas Soome <tsoome@me.com>
Closes #10774

include/os/freebsd/zfs/sys/Makefile.am

@@ -2,6 +2,7 @@ KERNEL_H = \
 	freebsd_crypto.h \
 	sha2.h \
 	vdev_os.h \
+	zfs_bootenv_os.h \
 	zfs_context_os.h \
 	zfs_ctldir.h \
 	zfs_dir.h \

include/os/freebsd/zfs/sys/zfs_bootenv_os.h

@@ -0,0 +1,29 @@
+/*
+ * This file and its contents are supplied under the terms of the
+ * Common Development and Distribution License ("CDDL"), version 1.0.
+ * You may only use this file in accordance with the terms of version
+ * 1.0 of the CDDL.
+ *
+ * A full copy of the text of the CDDL should have accompanied this
+ * source.  A copy of the CDDL is also available via the Internet at
+ * http://www.illumos.org/license/CDDL.
+ */
+
+/*
+ * Copyright 2020 Toomas Soome <tsoome@me.com>
+ */
+
+#ifndef _ZFS_BOOTENV_OS_H
+#define	_ZFS_BOOTENV_OS_H
+
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+#define	BOOTENV_OS		BE_FREEBSD_VENDOR
+
+#ifdef __cplusplus
+}
+#endif
+
+#endif /* _ZFS_BOOTENV_OS_H */