Make user keymaps fully declarative

This removes the need for the user to define... most things, honestly.
Notably, `main()` is no longer the end user's responsibility. This also
allows us to do fun stuff going forward like validating keymaps for
sanity (ex: the key assigned to `KC_MO(x)` should be assigned to
`KC_TRNS` on the target layer or the user will never be able to escape
that layer).

This also disambiguates `BOARD` to always refer to an actual slab of
silicon, renaming to `USER_KEYMAP`.

Entrypoints are now a bit more wild, and mostly-unsupported boards no
longer have working entrypoints. It's probably just time to scrap those
boards for now (until we have BLE HID and/or bitbang USB HID, at least).

kmk/entrypoints/handwire/pyboard.py

@@ -0,0 +1,24 @@
+import sys
+from logging import DEBUG
+
+from kmk.firmware import Firmware
+from kmk.micropython.pyb_hid import HIDHelper
+
+
+def main():
+    from kmk_keyboard_user import cols, diode_orientation, keymap, rows
+
+    try:
+        firmware = Firmware(
+            keymap=keymap,
+            row_pins=rows,
+            col_pins=cols,
+            diode_orientation=diode_orientation,
+            hid=HIDHelper,
+            log_level=DEBUG,
+        )
+
+        firmware.go()
+    except Exception as e:
+        sys.print_exception(e)
+        sys.exit(1)