image.bbclass: add DEB_{PRE, POST}PROCESS_COMMANDS to rootfs_command_variables list
Remove duplicate ROOTFS_POSTPROCESS_COMMAND in the rootfs_command_variables list.
Add DEB_PREPROCESS_COMMANDS and DEB_POSTPROCESS_COMMANDS to rootfs_command_variables
list for consistency with the RPM_ and OPKG_ versions of those variables.
Note: the package manager specific pre and post process commands
may removed entirely in Yocto 2.2 or later.
Signed-off-by: Bill Randle <william.c.randle@intel.com> Signed-off-by: Ross Burton <ross.burton@intel.com>
