The main reason is 0-day testing system which can directly
use these defconfigs for testing.

Enable support for all xilinx drivers which Microblaze
can use and disable dependency on external rootfs.cpio.
There is only one exception which is axi ethernet driver
which still uses NO_IRQ which is not defined for Microblaze.

Signed-off-by: Michal Simek <michal.simek@xilinx.com>