Correct a comment about overriding the SD_VERSION in CMakeLists.txt

[obra]

All existing boards that override SD_VERSION do it in board.mk and, at least, my naive attempt to do it in board.cmake didn't actually work.

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Description of Change

A doc change to help folks getting started with softdevice version customization.

[hathach]

the comment is correct though if you use

• cmake you change the board.cmake
• make you change the board.mk

These are 2 different bulid system. FYI, This repo started with make as official buildsystem, then I slowly add cmake as additional build system.

[obra]

Oh! I'm so sorry. I'd assumed that the make based build system was using the cmake build system under the covers, since the make targets were the ones docced in the README. I'll close this out.