[nrfconnect] Enable FPU and adjust stack sizes (#15797)
[nrfconnect] Enable FPU and adjust stack sizes (#15797)

FPU support is necessary to be able to use pre-compiled
OpenThread libraries. However, when FPU and stack guard
are enabled, BLE RX thread stack must be increased, too.
By the way, remove the OpenThread stack size setting as
it's set correctly, by default.
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