eepromread: support for reading from /dev/eeprom

eepromread could only read EEPROMs through the /dev/i2c interface.
Once the cat24c256 driver is started and claims/reserves the
device, it can no longer be read through the /dev/i2c interface.

This patch adds support for reading from EEPROMs through the
/dev/eeprom interface. For example, to read the on-board eeprom
on the BBB, one would do `eepromread -f /dev/eepromb1s50 -i`.

Change-Id: If08ce37231e593982eeb109bdd6d5458ad271108