treewide: move calibration data extraction function to library

treewide: move calibration data extraction function to library

This moves the almost identical calibration data extraction
functions present multiple times in several targets to a single
library file /lib/functions/caldata.sh.

Functions are renamed with more generic names to merge different
variants that only differ in their names.

Most of the targets used find_mtd_chardev, while some used
find_mtd_part inside the extraction code. To merge them, the more
abundant version with find_mtd_chardev is used in the common code.

Signed-off-by: Adrian Schmutzler <freifunk@adrianschmutzler.de>
[rebase on latest master; add mpc85xx]
Signed-off-by: David Bauer <mail@david-bauer.net>