common: add prototype & rename populate_serial_number()

Rename populate_serial_number() to a more descriptive
serial_read_from_eeprom() and provide the missing function prototype.

This is useful for boards that wish to read their serial number from
EEPROM at init.

Signed-off-by: Artur Rojek <artur@conclusive.pl>
Reviewed-by: Simon Glass <sjg@chromium.org>