Bluetooth: samples: hci_uart: Refactor conf files

Refactor conf files to use prj.conf + board/<board>.conf configuration.
This allows us to have put common configurations into the prj.conf and
have board specific configs in each board file.
This also respects adding additional prj.conf files such as
-DCONF_FILE='nrf5.conf debug.conf' to add debug configuration.

Signed-off-by: Joakim Andersson <joakim.andersson@nordicsemi.no>