wic: Add missing docstrings to plugin classes

Docstings from plugin classes are used as part of
output of 'wic help plugins'. Adding them makes help page
more informative.

(From OE-Core rev: d4414b45c81ab539f75a7bb16fc6412c30dfc45f)

Signed-off-by: Ed Bartosh <ed.bartosh@linux.intel.com>
Signed-off-by: Ross Burton <ross.burton@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

scripts/lib/wic/plugins/imager/direct_plugin.py

@@ -33,6 +33,15 @@ import wic.imager.direct as direct
 from wic.pluginbase import ImagerPlugin
 
 class DirectPlugin(ImagerPlugin):
+    """
+    Install a system into a file containing a partitioned disk image.
+
+    An image file is formatted with a partition table, each partition
+    created from a rootfs or other OpenEmbedded build artifact and dd'ed
+    into the virtual disk. The disk image can subsequently be dd'ed onto
+    media and used on actual hardware.
+    """
+
     name = 'direct'
 
     @classmethod

scripts/lib/wic/plugins/source/bootimg-efi.py

@@ -34,6 +34,11 @@ from wic.utils.oe.misc import exec_cmd, exec_native_cmd, get_bitbake_var, \
                               BOOTDD_EXTRA_SPACE
 
 class BootimgEFIPlugin(SourcePlugin):
+    """
+    Create EFI boot partition.
+    This plugin supports GRUB 2 and gummiboot bootloaders.
+    """
+
     name = 'bootimg-efi'
 
     @classmethod

scripts/lib/wic/plugins/source/bootimg-partition.py

@@ -32,6 +32,11 @@ from wic.utils.oe.misc import exec_cmd, get_bitbake_var
 from glob import glob
 
 class BootimgPartitionPlugin(SourcePlugin):
+    """
+    Create an image of boot partition, copying over files
+    listed in IMAGE_BOOT_FILES bitbake variable.
+    """
+
     name = 'bootimg-partition'
 
     @classmethod

scripts/lib/wic/plugins/source/bootimg-pcbios.py

@@ -34,6 +34,10 @@ from wic.utils.oe.misc import exec_cmd, exec_native_cmd, \
                               get_bitbake_var, BOOTDD_EXTRA_SPACE
 
 class BootimgPcbiosPlugin(SourcePlugin):
+    """
+    Create MBR boot partition and install syslinux on it.
+    """
+
     name = 'bootimg-pcbios'
 
     @classmethod

scripts/lib/wic/plugins/source/fsimage.py

@@ -22,6 +22,10 @@ from wic.pluginbase import SourcePlugin
 from wic.utils.oe.misc import get_bitbake_var
 
 class FSImagePlugin(SourcePlugin):
+    """
+    Add an already existing filesystem image to the partition layout.
+    """
+
     name = 'fsimage'
 
     @classmethod

scripts/lib/wic/plugins/source/rawcopy.py

@@ -22,6 +22,10 @@ from wic.pluginbase import SourcePlugin
 from wic.utils.oe.misc import exec_cmd, get_bitbake_var
 
 class RawCopyPlugin(SourcePlugin):
+    """
+    Populate partition content from raw image file.
+    """
+
     name = 'rawcopy'
 
     @classmethod

scripts/lib/wic/plugins/source/rootfs.py

@@ -32,6 +32,10 @@ from wic.pluginbase import SourcePlugin
 from wic.utils.oe.misc import find_bitbake_env_lines, find_artifact
 
 class RootfsPlugin(SourcePlugin):
+    """
+    Populate partition content from a rootfs directory.
+    """
+
     name = 'rootfs'
 
     @staticmethod