My intent was not to start a shell discussion, I would point you at a post I put on the OpenWrt forum years back, but well..
At any rate since you ask, what ever set it to:
CONFIG_BUSYBOX_CONFIG_FEATURE_EDITING_HISTORY=512
CONFIG_BUSYBOX_CONFIG_FEATURE_EDITING_SAVEHISTORY=y
CONFIG_BUSYBOX_CONFIG_FEATURE_EDITING_SAVE_ON_EXIT=y