Bluetooth: Offload ECC calculations to task

If TinyCrypt ECC is used for LE SC calculations need to be done
in task to avoid hogging CPU from non-preemptible fiber. To keep
upper layers of stack independent of crypto used (TinyCrypt or
controller) this patch adds HCI ECC emulation.

If ECC emulation is enabled it is always used regardless of ECC
support being present in controller.

Change-Id: I7c5ca873a18c10237e1c0b2f09e6da2a75fb334e
Origin: Original
Signed-off-by: Szymon Janc <ext.szymon.janc@tieto.com>