board/BuR/common: add 'brdefaulip_setup' function

Many B&R boards are equipped with an I2C-EEPROM where various
information can be stored.

Today there is only a single byte for 'board_id' used.

We write this 'board_id' into environment for later use during boot.

If the value != 0xFF, meaning the byte is programmed, we modify the
"brdefaultip" environment variable for setting an IP-Address based on
board_id.

Signed-off-by: Hannes Schmelzer <hannes.schmelzer@br-automation.com>