raspberypi: Allow mpconfigboard to override additional items

This allows more options to be overridden to 0 in an mpconfigboard.mk.

Improved:
 * FRAMEBUFFERIO, FULL_BUILD, BITOPS, PWMIO, RGBMATRIX, OTARYIO, PULSEIO, WATCHDOG

Still problematic (pull requests welcome):
 * RP2PIO & NEOPIXEL_WRITE, possibly only if a status neopixel is defined
 * BITBANGIO, possibly only if BUSIO is enabled
 * RTC

Incidentally, with RP2PIO & NEOPIXEL_WRITE, BITBANGIO, and RTC re-enabled I get
```
323956 bytes used, 720524 bytes free in flash firmware space out of 1044480 bytes (1020.0kB).
12072 bytes used, 250072 bytes free in ram for stack and heap out of 262144 bytes (256.0kB).
```

Closes #4515

Author: jepler

ports/raspberrypi/mpconfigport.mk

@@ -17,28 +17,24 @@ endif
 # All raspberrypi ports have longints.
 LONGINT_IMPL = MPZ
 
-ifndef CIRCUITPY_RP2PIO
-CIRCUITPY_RP2PIO = 1
-else
-CIRCUITPY_NEOPIXEL_WRITE = 0
-endif
-
-CIRCUITPY_FRAMEBUFFERIO = 1
-CIRCUITPY_FULL_BUILD = 1
+CIRCUITPY_RP2PIO ?= 1
+CIRCUITPY_NEOPIXEL_WRITE ?= $(CIRCUITPY_RP2PIO)
+CIRCUITPY_FRAMEBUFFERIO ?= 1
+CIRCUITPY_FULL_BUILD ?= 1
 CIRCUITPY_AUDIOMP3 ?= 1
-CIRCUITPY_BITOPS = 1
-CIRCUITPY_PWMIO = 1
-CIRCUITPY_RGBMATRIX = 1
-CIRCUITPY_ROTARYIO = 1
+CIRCUITPY_BITOPS ?= 1
+CIRCUITPY_PWMIO ?= 1
+CIRCUITPY_RGBMATRIX ?= 1
+CIRCUITPY_ROTARYIO ?= 1
 
 # Things that need to be implemented.
 # Use PWM interally
 CIRCUITPY_FREQUENCYIO = 0
 CIRCUITPY_I2CPERIPHERAL = 0
 CIRCUITPY_NVM = 1
 # Use PIO interally
-CIRCUITPY_PULSEIO = 1
-CIRCUITPY_WATCHDOG = 1
+CIRCUITPY_PULSEIO ?= 1
+CIRCUITPY_WATCHDOG ?= 1
 
 # Audio via PWM
 CIRCUITPY_AUDIOIO = 0