README: Review the u-boot porting guide list

* There is no boards.cfg anymore, so drop (1).
* Creating flash.c and u-boot.lds seems not mandatory as well.
* Adjusting the enumerators for the above implicitly fixed for
  double items numbered (3).

Signed-off-by: Phil Sutter <phil@nwl.cc>
Acked-by: Stefan Roese <sr@denx.de>
Reviewed-by: Tom Rini <trini@konsulko.com>