Add soft device rng portion to MicroBitDevice::seedRandom(). #330

Job	Run time
ubuntu-20.04, gcc 7-2017-q4, cmake 3.6.0	46s
ubuntu-20.04, gcc 7-2017-q4, cmake latest	43s
ubuntu-20.04, gcc latest, cmake 3.6.0	50s
ubuntu-20.04, gcc latest, cmake latest	51s
macos-12, gcc 7-2017-q4, cmake 3.6.0	1m 30s
macos-12, gcc 7-2017-q4, cmake latest	1m 19s
macos-12, gcc latest, cmake 3.6.0	1m 36s
macos-12, gcc latest, cmake latest	2m 9s
windows-2019, gcc 7-2017-q4, cmake 3.6.0	2m 7s
windows-2019, gcc 7-2017-q4, cmake latest	3m 13s
windows-2019, gcc latest, cmake 3.6.0	2m 17s
windows-2019, gcc latest, cmake latest	3m 58s
	21m 19s

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