Initial merge of Nordic Semi nRF51/52 from Unicore MX back into Libopencm3

* merged: nrf tree from unicore-mx
* fixed: small changes to make merged code play with rest of locm3 again
* added: linker script generator defines for nRF51/52 stubs
* added: doxygen support

This removes code and changes names and styles where relevant to be more
inline with normal libopencm3.

NRF52x library is built for hardfloat, M4F by default.  The M4 no float
variants are less common, and if needed, the library can be built
manually for those variants.  Unless some very common boards show up
using those parts, we don't need an extra library build.

Reviewed-by: Karl Palsson <karlp@tweak.net.au>
Tested-by: Karl Palsson <karlp@tweak.net.au>

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