Add soft device rng portion to MicroBitDevice::seedRandom(). #328

Job	Run time
ubuntu-20.04, gcc 7-2017-q4, cmake 3.6.0	1m 2s
ubuntu-20.04, gcc 7-2017-q4, cmake latest	1m 12s
ubuntu-20.04, gcc latest, cmake 3.6.0	1m 22s
ubuntu-20.04, gcc latest, cmake latest	1m 26s
macos-12, gcc 7-2017-q4, cmake 3.6.0	1m 50s
macos-12, gcc 7-2017-q4, cmake latest	1m 58s
macos-12, gcc latest, cmake 3.6.0	2m 39s
macos-12, gcc latest, cmake latest	3m 18s
windows-2019, gcc 7-2017-q4, cmake 3.6.0	2m 39s
windows-2019, gcc 7-2017-q4, cmake latest	3m 4s
windows-2019, gcc latest, cmake 3.6.0	2m 42s
windows-2019, gcc latest, cmake latest	2m 57s
	26m 9s

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