ipc: icmsg: configure PBUF RX rx_buffer using kconfig

The size of the rx_buffer used by icmsg.c mbox_callback_process()
is not MISRA compliant, being "dynamically" allocated on the stack.

This commit adds a kconfig to set the size of the read buffer and
asserts that it is large enough to read all received data.

Signed-off-by: Bjarki Arge Andreasen <bjarki.andreasen@nordicsemi.no>