Bluetooth: Set hci_uart RAM config to fit privacy

With the introduction of privacy, the system workqueue is now being
linked into the image, growing the RAM footprint considerably.

In order for the sample to fit in QFAA and QFAB variants of the nRF51
(16KB of RAM) we reduce the workqueue stack size to 512 since the RPA
update mechanism doesn't use more than 340 bytes and we stick to 16
connections maximum.

Signed-off-by: Carles Cufi <carles.cufi@nordicsemi.no>