shrink some samd21 boards #17184

Job	Run time
scheduler	44s
docs	13m 45s
mpy-cross-mac	1m 23s
windows	0s
windows-zephyr	0s
tests / run (mpy)	45s
tests / run (native)	46s
tests / run (all)	1m 6s
tests / run (native_mpy)	40s
mpy-cross / build (static)	25s
mpy-cross / build (static-aarch64)	6m 1s
mpy-cross / build (static-mingw)	48s
mpy-cross / build (static-raspbian)	44s
ports (atmel-samd) / board (feather_m0_supersized)	5m 56s
ports (atmel-samd) / board (snekboard)	5m 45s
	38m 48s

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