bcmstb: Migrate CONFIG_CMD_EXT[24]

The common config header for bcm7260 and bcm7445 enable
CONFIG_CMD_EXT2/EXT4 but the defconfigs do not.  This results in the
commands being disabled.  Enable them via the defconfig.

Cc: Thomas Fitzsimmons <fitzsim@fitzsim.org>
Signed-off-by: Tom Rini <trini@konsulko.com>