Initramfs scripts for ZoL.

* Supports booting of a ZFS snapshot.
  Do this by cloning the snapshot into a dataset. If this, the resulting
  dataset, already exists, destroy it. Then mount it on root.
  * If snapshot does not exist, use base dataset (the part before '@')
    as boot filesystem instead.
  * If no snapshot is specified on the 'root=' kernel command line, but there
    is an '@', then get a list of snapshots below that filesystem and ask the
    user which to use.
  * Clone with 'mountpoint=none' and 'canmount=noauto' - we mount manually
    and explicitly.
    * For sub-filesystems, that doesn't have a mountpoint property set, we use
      the 'org.zol:mountpoint' to keep track of it's mountpoint.
  * Allow rollback of snapshots instead of clone it and boot from the clone.
* Allow mounting a root- and subfs with mountpoint=legacy set
* Allow mounting a filesystem which is using nativ encryption.
* Support all currently used kernel command line arguments
  All the different distributions have their own standard on what to specify
  on the kernel command line to boot of a ZFS filesystem.
  * Extra options:
    * zfsdebug=(on,yes,1)	Show extra debugging information
    * zfsforce=(on,yes,1)	Force import the pool
    * rollback=(on,yes,1)	Rollback (instead of clone) the snapshot
* Only try to import pool if it haven't already been imported
  * This will negate the need to force import a pool that have not been exported cleanly.
  * Support exclusion of pools to import by setting ZFS_POOL_EXCEPTIONS in /etc/default/zfs.
* Support additional configuration variable ZFS_INITRD_ADDITIONAL_DATASETS
  to mount additional filesystems not located under your root dataset.
* Include /etc/modprobe.d/{zfs,spl}.conf in the initrd if it/they exist.
* Include the udev rule to use by-vdev for pool imports.
* Include the /etc/default/zfs file to the initrd.
* Only try /dev/disk/by-* in the initrd if USE_DISK_BY_ID is set.
  * Use /dev/disk/by-vdev before anything.
  * Add /dev as a last ditch attempt.
  * Fallback to using the cache file if that exist if nothing else worked.
* Use /sbin/modprobe instead of built-in (BusyBox) modprobe.
  This gets rid of the message "modprobe: can't load module zcommon".
  Thanx to pcoultha for finding this.

Signed-off-by: Turbo Fredriksson <turbo@bayour.com>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #2116
Closes #2114

config/zfs-build.m4

@@ -139,7 +139,7 @@ AC_DEFUN([ZFS_AC_RPM], [
 	])
 
 	RPM_DEFINE_COMMON='--define "$(DEBUG_ZFS) 1" --define "$(DEBUG_DMU_TX) 1"'
-	RPM_DEFINE_UTIL='--define "_dracutdir $(dracutdir)" --define "_udevdir $(udevdir)" --define "_udevruledir $(udevruledir)" --define "_initconfdir $(DEFAULT_INITCONF_DIR)"'
+	RPM_DEFINE_UTIL='--define "_dracutdir $(dracutdir)" --define "_udevdir $(udevdir)" --define "_udevruledir $(udevruledir)" --define "_initconfdir $(DEFAULT_INITCONF_DIR)" $(DEFINE_INITRAMFS)'
 	RPM_DEFINE_KMOD='--define "kernels $(LINUX_VERSION)" --define "require_spldir $(SPL)" --define "require_splobj $(SPL_OBJ)" --define "ksrc $(LINUX)" --define "kobj $(LINUX_OBJ)"'
 	RPM_DEFINE_DKMS=
 
@@ -285,7 +285,6 @@ AC_DEFUN([ZFS_AC_DEFAULT_PACKAGE], [
 		debian)     DEFAULT_PACKAGE=deb  ;;
 		*)          DEFAULT_PACKAGE=rpm  ;;
 	esac
-
 	AC_MSG_RESULT([$DEFAULT_PACKAGE])
 	AC_SUBST(DEFAULT_PACKAGE)
 
@@ -308,7 +307,6 @@ AC_DEFUN([ZFS_AC_DEFAULT_PACKAGE], [
 		debian)     DEFAULT_INIT_SCRIPT=lsb    ;;
 		*)          DEFAULT_INIT_SCRIPT=lsb    ;;
 	esac
-
 	AC_MSG_RESULT([$DEFAULT_INIT_SCRIPT])
 	AC_SUBST(DEFAULT_INIT_SCRIPT)
 
@@ -323,9 +321,18 @@ AC_DEFUN([ZFS_AC_DEFAULT_PACKAGE], [
 		debian)     DEFAULT_INITCONF_DIR=/etc/default   ;;
 		*)          DEFAULT_INITCONF_DIR=/etc/default   ;;
 	esac
-
 	AC_MSG_RESULT([$DEFAULT_INITCONF_DIR])
 	AC_SUBST(DEFAULT_INITCONF_DIR)
+
+	AC_MSG_CHECKING([whether initramfs-tools is available])
+	if test -d /usr/share/initramfs-tools ; then
+		DEFINE_INITRAMFS='--define "_initramfs 1"'
+		AC_MSG_RESULT([yes])
+	else
+		DEFINE_INITRAMFS=''
+		AC_MSG_RESULT([no])
+	fi
+	AC_SUBST(DEFINE_INITRAMFS)
 ])
 
 dnl #