Revert "Bluetooth: Remove LE Rand workaround for mynewt firmware"

The current documentation points to MyNewt 0.9 which doesn't have the
necessary fix, so revert the workaround until the next release of
MyNewt is out. A minor change to the original patch is that we also
have a delay in prng_init() which seems to be needed since this is now
called earlier during the HCI init sequence.

This reverts commit d75acb44.

Change-Id: Ic4919e01bddbfaca477825c3f7fdcb549c8d10ec
Signed-off-by: Johan Hedberg <johan.hedberg@intel.com>