am335x_evm: Enable CONFIG_CMD_BOOTZ

With v3.9 and later of the Linux Kernel defaulting to multi-platform
images with omap2plus_defconfig, uImage isn't builtable anymore by
default.  Add CONFIG_CMD_BOOTZ so that we can still boot something the
kernel spits out.

Signed-off-by: Tom Rini <trini@ti.com>
Acked-by: Peter Korsgaard <jacmet@sunsite.dk>