discovery: ignore short ATND responses
samples/gpm: remove non-existent "scan" command from help menu
samples/gpm: use xbee_gpm_max_write() to set default page size
device: add new Modem Status values for XBee S6B (Wi-Fi)
samples/gpm: remove workaround for bug in early XBee S6B releases; firmware 2020 and later accept self-addressed frames
doc: update README.md