base-files: add minimal mmc support

base-files: add minimal mmc support

Added minimal mmc support for helper functions:

 - find_mmc_part: Look for a given partition name. Returns the
	coresponding partition path
 - caldata_extract_mmc: Look for a given partition name and then
	extracts the calibration data
 - mmc_get_mac_binary: Returns the mac address from a given partition
	name and offset

Signed-off-by: Davide Fioravanti <pantanastyle@gmail.com>
Signed-off-by: Robert Marko <robimarko@gmail.com>
[replace dd with caldata_dd, moved sysupgrade mmc to orbi]
Signed-off-by: Christian Lamparter <chunkeey@gmail.com>