zephyr: Enable SLIP networking for the default build.

This makes MicroPython app running in QEMU be pingable from the host (by
following QEMU networking setup instructions,
https://www.zephyrproject.org/doc/samples/net/qemu_setup.html).

zephyr/main.c

@@ -28,6 +28,12 @@
 #include <stdio.h>
 #include <string.h>
 
+#include <zephyr.h>
+#ifdef CONFIG_NETWORKING
+#include <net/net_context.h>
+#include <net/nbuf.h>
+#endif
+
 #include "py/nlr.h"
 #include "py/compile.h"
 #include "py/runtime.h"
@@ -60,6 +66,14 @@ void do_str(const char *src, mp_parse_input_kind_t input_kind) {
 static char *stack_top;
 static char heap[MICROPY_HEAP_SIZE];
 
+void init_zephyr(void) {
+    #ifdef CONFIG_NET_IPV4
+    // TODO: Make address configurable
+    static struct in_addr in4addr_my = { { { 192, 0, 2, 1 } } };
+    net_if_ipv4_addr_add(net_if_get_default(), &in4addr_my, NET_ADDR_MANUAL, 0);
+    #endif
+}
+
 int real_main(void) {
     int stack_dummy;
     stack_top = (char*)&stack_dummy;
@@ -67,6 +81,8 @@ int real_main(void) {
     // Make MicroPython's stack limit somewhat smaller than full stack available
     mp_stack_set_limit(CONFIG_MAIN_STACK_SIZE - 512);
 
+    init_zephyr();
+
 soft_reset:
     #if MICROPY_ENABLE_GC
     gc_init(heap, heap + sizeof(heap));

zephyr/prj.conf

@@ -4,3 +4,16 @@ CONFIG_UART_CONSOLE_DEBUG_SERVER_HOOKS=y
 CONFIG_NEWLIB_LIBC=y
 CONFIG_FLOAT=y
 CONFIG_MAIN_STACK_SIZE=4096
+
+# Networking config
+CONFIG_NETWORKING=y
+CONFIG_NET_IPV4=y
+CONFIG_TEST_RANDOM_GENERATOR=y
+CONFIG_NET_NBUF_RX_COUNT=4
+
+# Networking drivers
+# SLIP driver for QEMU
+CONFIG_NET_SLIP_TAP=y
+
+# BOARD-specific config (qemu_x86)
+CONFIG_RAM_SIZE=256