zephyr: Enable usb mass storage class on mimxrt1050_evk.

Enables the zephyr usb device stack and mass storage class on the
mimxrt1050_evk board. The mass storage class is backed by the sdhc disk
access driver, so it's now possible to browse and modify the contents of
the SD card from a USB host (your PC). This is in preparation to support
writing a main.py script to the SD card, and then executing it after the
next reset.

ports/zephyr/main.c

@@ -34,6 +34,10 @@
 #include <net/net_context.h>
 #endif
 
+#ifdef CONFIG_USB
+#include <usb/usb_device.h>
+#endif
+
 #include "py/mperrno.h"
 #include "py/compile.h"
 #include "py/runtime.h"
@@ -133,6 +137,10 @@ soft_reset:
     mp_obj_list_append(mp_sys_path, MP_OBJ_NEW_QSTR(MP_QSTR_)); // current dir (or base dir of the script)
     mp_obj_list_init(mp_sys_argv, 0);
 
+    #ifdef CONFIG_USB
+    usb_enable(NULL);
+    #endif
+
     #if MICROPY_VFS
     vfs_init();
     #endif

ports/zephyr/prj_mimxrt1050_evk.conf

@@ -1,3 +1,9 @@
 # Required for zephyr.DiskAccess block devices
 CONFIG_DISK_ACCESS=y
 CONFIG_DISK_ACCESS_SDHC=y
+
+CONFIG_USB=y
+CONFIG_USB_DEVICE_STACK=y
+CONFIG_USB_DEVICE_PRODUCT="Zephyr MicroPython"
+CONFIG_USB_MASS_STORAGE=y
+CONFIG_MASS_STORAGE_DISK_NAME="SDHC"