ti: common: board_detect: commodify ethaddr environment setting code

Keystone and OMAP platforms will need this to set ethernet
MAC addresses from board EEPROM.

Signed-off-by: Roger Quadros <rogerq@ti.com>
Reviewed-by: Lokesh Vutla <lokeshvutla@ti.com>
Reviewed-by: Tom Rini <trini@konsulko.com>