base-files version

opkg list-installed base-files | awk -v FS="-" '{print $4}'

/etc/banner (may fail if not trunk)

awk -v FS="[ )]" '/r[0-9]+/ {print $5}' /etc/banner

/etc/banner

awk 'match($0,/r[0-9]+/) {print substr($0,RSTART,RLENGTH)}' /etc/banner

All of the above outputs the same:

r25094

Any shorter method?