kernel: add KERNEL_IO_URING option

kernel: add KERNEL_IO_URING option

* add KERNEL_IO_URING option

NOTES:
Adds configurable support for the io_uring interface (CONFIG_IO_URING) via KERNEL_IO_URING option.

The kernel only zImage grows by about 5-9KB ?

I would like to enable this by default for all 5.4 kernels, so i can use the new io_uring samba-4.12.x vfs module by default.

The associated liburing was already submitted and merged.
The kernel + liburing was tested on ARM/mvebu via samba4 vfs_io_uring module and i have no issues so far.

Some extra reads on it and why we should enable it by default, since i expect more packages to use this in the future.
https://wiki.samba.org/index.php/Samba_4.12_Features_added/changed#.27io_uring.27_vfs_module
https://lwn.net/Articles/810414/
https://kernel.dk/io_uring.pdf
https://www.phoronix.com/scan.php?page=news_item&px=Linux-5.6-IO-uring-Tests

Signed-off-by: Andy Walsh <andy.walsh44+github@gmail.com>