Bluetooth: Kconfig: Fix BT_ECC defaults

The commit e85dd8af changed the way the BT_ECC Kconfig option
is enabled, however it got the dependency wrong. The dependency should
only look at BT_SMP_OOB_LEGACY_PAIR_ONLY if BT_SMP was also enable.

This broke e.g. the build of the mesh_demo app for the BBC
micro:bit since the memory consumption jumped up by roughly 2k.

This patch fixes the issue, and in the same go makes the Mesh handling
consistent by also using a conditional default rather than select.

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