The RAM base address is needed during SoftDevice initialization. So far,
the same magic value has been used in aykevl/go-bluetooth and in TinyGo,
but this should be configured in only one place.
This will have additional benefits in the future:
* It is currently set to 0x39c0, which is around 14.5kB. Most nrf51822
chips have only 16kB of RAM, so this is way too much for those
chips.
* LLD in LLVM 11 allows expressions in the MEMORY part of linker
scripts, which will allow overriding the SoftDevice RAM area with a
linker flag, which might come in handy.
