NRF51 with mbed OS (5) and RawSerial test

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0:08d373203e2e
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+++ b/mbed-os.lib	Mon Jan 23 09:46:18 2017 +0000
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+https://github.com/ARMmbed/mbed-os/#34c1facf42a174f47fdf9002cd8c6bf10ac41744