target: Add target_core_backend_configfs.h helper macros

This patch adds a number of configfs e-attr macros following
what existing target_core_configfs.c code does for internal
target_backend_dev_attrib setup, and similar to how target
fabric drivers allow for external config_item_type + cit->ct_attrs.
assignment.

This is useful for backend drivers like PSCSI who need to only
expose a small subset of device attributes, while still retaining
a default list of attributes for other backend drivers like
IBLOCK, FILEIO, RAMDISK, and TCMU.

Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>