common/env_flags.c: Add function to validate a MAC address

common/env_flags.c: Add function to validate a MAC address

The code that checks if a string has the format of a MAC address has been
moved to a separate function called eth_validate_ethaddr_str().

This has been done to allow other components (such as vsc9953 driver)
to validate a MAC address.

Signed-off-by: Codrin Ciubotariu <[email protected]>
Acked-by: Joe Hershberger <[email protected]>
Reviewed-by: York Sun <[email protected]>