ipc: openamp: Expose config options for MASTER/SLAVE builds

We can build the openamp library configured with VirtIO master
support, VirtIO slave support, or both.  By default both master
and slave code is enabled.  We can reduce code footprint by only
build master or slave as needed.

Expose Kconfig options for Master & Slave and set them accordingly in
the sample.

Here's the code reduction we see:

For the total image we see as 1260 byte reduction:
Memory region         Used Size  Region Size  %age Used
 FLASH [Master & Slave]:       30308 B       256 KB     11.56%
 FLASH [Master only]   :       29048 B       256 KB     11.08%

On the remote side we see a 828 byte reduction:

Memory region         Used Size  Region Size  %age Used
 FLASH [Master & Slave]:       11564 B        64 KB     17.65%
 FLASH [Slave only]    :       10736 B        64 KB     16.38%

Signed-off-by: Kumar Gala <kumar.gala@linaro.org>