The following variables now expect a semicolon separated list of functions to call and not arbitrary shell commands:
ROOTFS_PREPROCESS_COMMAND ROOTFS_POSTPROCESS_COMMAND SDK_POSTPROCESS_COMMAND POPULATE_SDK_POST_TARGET_COMMAND POPULATE_SDK_POST_HOST_COMMAND IMAGE_POSTPROCESS_COMMAND IMAGE_PREPROCESS_COMMAND ROOTFS_POSTUNINSTALL_COMMAND ROOTFS_POSTINSTALL_COMMAND
For migration purposes, you can simply wrap shell commands in a shell function and then call the function. Here is an example:
my_postprocess_function() { echo "hello" > ${IMAGE_ROOTFS}/hello.txt } ROOTFS_POSTPROCESS_COMMAND += "my_postprocess_function; "