configs: am335x_evm: Enable Fastboot

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:

    => fastboot 0
    $ fastboot flash rootfs rootfs.img

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