zephyr: Introduce Zephyr native filesystem VFS interface.

This allows using Zephyr's supported VFS and interface it to MicroPython's
VFS.  That makes the MicroPython-side filesystem drivers unecessary, eg you
can disable LFS2 in MicroPython but still do LFS2 because it's using
Zephyr's.

An advantage of this is that the filesystem can be shared between Zephyr C
code and MicroPython.

Signed-off-by: Vdragon <mail@massdriver.space>

ports/zephyr/CMakeLists.txt

@@ -55,6 +55,7 @@ set(MICROPY_SOURCE_PORT
     uart_core.c
     zephyr_device.c
     zephyr_storage.c
+    zephyr_filesystem.c
     mpthreadport.c
 )
 list(TRANSFORM MICROPY_SOURCE_PORT PREPEND ${MICROPY_PORT_DIR}/)