configs: am335x_evm: Enable DFU over TFTP support

DFU over TFTP flashing method might be helpful in order to speed-up the
flashing process of big images (as DFU works over USB EP0, which is
quite slow). Also, it's a good backup option in the case when USB got
broken (either in software or hardware), to avoid resorting to SD card
boot.

This config option was present in am335x_boneblack_defconfig, but we
have to use generic am335x_evm_defconfig now, as BBB defconfig was
removed in commit 8fa7f65d ("configs: Remove
am335x_boneblack_defconfig"). So this patch merely brings that option
back.

Tested on BeagleBone Black:

    => setenv dfu_alt_info $dfu_alt_info_emmc
    => setenv ipaddr 192.168.0.100
    => setenv serverip 192.168.0.1
    => setenv updatefile update_uboot.itb
    => dfu tftp mmc 1

Where 'update_uboot.its' file looks like this:

    /dts-v1/;

    / {
        description = "Automatic U-Boot update";
        #address-cells = <1>;

        images {
            u-boot.img.raw-1 {
                description = "U-Boot binary";
                data = /incbin/("u-boot.img");
                compression = "none";
                type = "firmware";
                load = <0x60000>;
                hash-1 {
                    algo = "sha1";
                };
            };
        };
    };

And 'update_uboot.itb' is generated as follows:

    $ mkimage -f update_uboot.its update_uboot.itb

Newly flashed U-Boot works fine.

Fixes: 8fa7f65d ("configs: Remove am335x_boneblack_defconfig")
Signed-off-by: Sam Protsenko <joe.skb7@gmail.com>