zephyr: Use CONFIG_USB_DEVICE_STACK for conditional USB device support.

CONFIG_USB was removed in Zephyr v2.7.0 after some Kconfig rework that
made it sufficient to use CONFIG_USB_DEVICE_STACK only.

Signed-off-by: Maureen Helm <maureen.helm@intel.com>

ports/zephyr/boards/mimxrt1050_evk.conf

@@ -1,7 +1,6 @@
 # Required for zephyr.DiskAccess block devices
 CONFIG_DISK_ACCESS=y
 
-CONFIG_USB=y
 CONFIG_USB_DEVICE_STACK=y
 CONFIG_USB_DEVICE_PRODUCT="Zephyr MicroPython"
 CONFIG_USB_MASS_STORAGE=y

ports/zephyr/main.c

@@ -34,7 +34,7 @@
 #include <net/net_context.h>
 #endif
 
-#ifdef CONFIG_USB
+#ifdef CONFIG_USB_DEVICE_STACK
 #include <usb/usb_device.h>
 #endif
 
@@ -140,7 +140,7 @@ soft_reset:
     #endif
     mp_init();
 
-    #ifdef CONFIG_USB
+    #ifdef CONFIG_USB_DEVICE_STACK
     usb_enable(NULL);
     #endif