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.
Introduces a new zephyr.DiskAccess class that uses the zephyr disk
access api to implement the uos.AbstractBlockDev protocol. This can be
used with any type of zephyr disk access driver, which currently
includes SDHC, RAM, and FLASH implementations. The SDHC driver is
enabled on the mimxrt1050_evk board.
Only the standard block device protocol (without the offset parameter)
can be supported with the zephyr disk access api, therefore this class
cannot be used with file systems like littlefs which require the
extended interface. In the future it may be possible to implement the
extended interface in a new class using the zephyr flash api.
