The system-wide backup boot release file name, with extension '.rel'.
A zero-length string indicates that there is no backup release file so
the primary release file (juniSystemBootReleaseFile) will be used. On a
set operation if there is no file found with the name specified, then an
error is returned. The juniSystemBootBackupConfigControl object with a
valid value other than none(2) must be specified together with this
object in the same set request PDU, and if the
juniSystemBootBackupConfigControl is set to file(0) then a valid
juniSystemBootBackupConfigFile must also be included in the set request
PDU.