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>