These can be used to insert arbitrary checks, polling, etc into the VM.
They are left general because the VM is a highly tuned loop and it should
be up to a given port how that port wants to modify the VM internals.

One common use would be to insert a polling check, but only done after
a certain number of opcodes were executed, so as not to slow down the VM
too much.  For example:

 #define MICROPY_VM_HOOK_COUNT (30)
 #define MICROPY_VM_HOOK_INIT static uint vm_hook_divisor = MICROPY_VM_HOOK_COUNT
 #define MICROPY_VM_HOOK_POLL if (--vm_hook_divisor == 0) { \
     vm_hook_divisor = MICROPY_VM_HOOK_COUNT;
     extern void vm_hook_function(void);
     vm_hook_function();
 }
 #define MICROPY_VM_HOOK_LOOP MICROPY_VM_HOOK_POLL
 #define MICROPY_VM_HOOK_RETURN MICROPY_VM_HOOK_POLL

- As described in the #1850.
- Add cmdline tests.

- Put the I2C bus on the corect pins
- Add the appropriate board_init to power the shield

TODO: Probably merge into vfs_fat_diskio.

If None was returned for such requests (which likely means that user simply
didn't handle them), it means successful init and default sector size of 512
bytes respectively. This makes only BP_IOCTL_SEC_COUNT a mandatory request,
and thus re-establishes parity with old interface, where only .count() is
mandatory().

This implements OO interface based on existing fsusermount code and with
minimal changes to it, to serve as a proof of concept of OO interface.

Examle of usage:

bdev = RAMFS(48)
uos.VfsFat.mkfs(bdev)
vfs = uos.VfsFat(bdev, "/ramdisk")
f = vfs.open("foo", "w")
f.write("hello!")
f.close()

We have so many configuration options, that finally having shortcuts like
this is helpful and cuts on number of ifdef's.

This groups the pins for a port together and puts them in numerical
order. It also adds ARDUINO pin names.

mp_hal_gpio_set_af will search for a given function and unit
and set the alternate function to the alternate function index
found.

This patch adds support to fsusermount for multiple block devices
(instead of just one).  The maximum allowed is fixed at compile time by
the size of the fs_user_mount array accessed via MP_STATE_PORT, which
in turn is set by MICROPY_FATFS_VOLUMES.

With this patch, stmhal (which is still tightly coupled to fsusermount)
is also modified to support mounting multiple devices   And the flash and
SD card are now just two block devices that are mounted at start up if
they exist (and they have special native code to make them more
efficient).