ath79: move mikrotik-caldata.sh to target base-files

ath79: move mikrotik-caldata.sh to target base-files

Mikrotik devices will be found in both generic and nand subtargets.

The file mikrotik-caldata.sh, currently used in generic, contains
a few lines of code that would need to be duplicated for nand
support. Instead of duplicating it, move it to target base-files,
as size impact is small and the maintenance gain should outweigh it.

This is changed separately to make life easier for the people
currently working on Mikrotik NAND support.

Signed-off-by: Adrian Schmutzler <freifunk@adrianschmutzler.de>