zfs: Add ZFS filesystem support

U-Boot port is based on sources forked from GRUB-0.97 by Sun in 2004,
which can be found here:
http://src.opensolaris.org/source/xref/onnv/onnv-gate/usr/src/grub/grub-0.97/stage2/zfs-include/zfs.h

Released by Sun for GRUB under the license:
  *  This program is free software; you can redistribute it and/or modify
  *  it under the terms of the GNU General Public License as published by
  *  the Free Software Foundation; either version 2 of the License, or
  *  (at your option) any later version.

GRUB official releases include ZFS in version:
ftp://alpha.gnu.org/gnu/grub/grub-1.99~rc1.tar.gz

And patched against GRUB Bazaar repository for ashift fixes (4KB HDDs)
more conveniently found at github:
https://github.com/pendor/grub-zfs/commit/e7b6ef3ac3b9685ac4c394c897b1d4221b7381f1

Signed-off-by: Jorgen Lundman <lundman@lundman.net>
diff --git a/common/Makefile b/common/Makefile
index 483eb4d..3d62775 100644
--- a/common/Makefile
+++ b/common/Makefile
@@ -162,6 +162,7 @@
 COBJS-$(CONFIG_CMD_XIMG) += cmd_ximg.o
 COBJS-$(CONFIG_YAFFS2) += cmd_yaffs2.o
 COBJS-$(CONFIG_CMD_SPL) += cmd_spl.o
+COBJS-$(CONFIG_CMD_ZFS) += cmd_zfs.o
 
 # others
 ifdef CONFIG_DDR_SPD