kernel: make kmod-i2c-core selected by dependent modules

Currently kmod-i2c-* will not get into images unless kmod-i2c-core is added to
DEVICE_PACKAGES as well. By changing the dependencies from "depends on" to
"select", we do not have the issue anymore.

Furthermore, we can remove most occurrences of the package from DEVICE_PACKAGES
and similar variables, as it is now pulled by dependent modules such as:
- kmod-hwmon-lm75
- kmod-i2c-gpio
- kmod-i2c-gpio-custom
- kmod-i2c-mux
- kmod-i2c-ralink

Signed-off-by: Sungbo Eo <mans0n@gorani.run>
[do not touch ar71xx]
Signed-off-by: Adrian Schmutzler <freifunk@adrianschmutzler.de>