emul: Don't panic if matching emul can't be found

When initializing emulators for devices registered on an emulated bus,
Zephyr will assert if a matching emulator for the device cannot be
found. This feels overly restrictive --there may be cases where we still
want to build a driver for testing even without an emulator and drivers
should be able to handle situations where there is no device emulator
present (the I2C/SPI transactions will simply fail and the driver never
becomes ready).

This commit removes the assert and replaces it with an warning message if
no matching emulator is found.

Signed-off-by: Tristan Honscheid <honscheid@google.com>

subsys/emul/emul.c

@@ -40,7 +40,10 @@ int emul_init_for_bus(const struct device *dev)
 	for (elp = cfg->children; elp < end; elp++) {
 		const struct emul *emul = emul_get_binding(elp->dev->name);
 
-		__ASSERT(emul, "Cannot find emulator for '%s'", elp->dev->name);
+		if (!emul) {
+			LOG_WRN("Cannot find emulator for '%s'", elp->dev->name);
+			continue;
+		}
 
 		switch (emul->bus_type) {
 		case EMUL_BUS_TYPE_I2C: