Platform/RPi3: Add TFTP command to UEFI Shell

TFTP support, which is disabled by default, can be enabled by setting
INCLUDE_TFTP_COMMAND to TRUE.

Signed-off-by: Pete Batard <pete@akeo.ie>
Reviewed-by: Philippe Mathieu-Daude <philmd@redhat.com>
Reviewed-by: Leif Lindholm <leif.lindholm@linaro.org>