gemini: Use library functions to assign MAC address

gemini: Use library functions to assign MAC address

This simplifies the ethernet address extraction script
by using standard library functions to locate the MTD
partitions and extract ethernet address from a binary
offset location in the flash. Furthermore, the aging
ifconfig is replaced by the ip tool, which will now
assign the MAC addresses.

Suggested-by: Christian Lamparter <chunkeey@gmail.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Christian Lamparter <chunkeey@gmail.com>
[ifconfig replacement, use -n instead of ! -z,
-n requires string to be quoted within the test brackets,
drop prepended "x" in check, add quotes, make local
variables local, kill whitespaces]

(cherry picked from commit cd6c3535cd2c7779a8d65f792c36082ee3822c11)