ppc4xx: Add CONFIG_BOOTP_SUBNETMASK to Sequoia board config

When using dhcp/bootp the "netmask" environment variable is not
set because CONFIG_BOOTP_SUBNETMASK is not defined. But usually this is
desireable, so the following patch adds this this option to the board
config.

Signed-off-by: Markus Klotzbuecher <mk@denx.de>
Signed-off-by: Stefan Roese <sr@denx.de>