logging: backend: uart: append index conditionally for compatibility

Updated the `LBU_DEFINE` so that the index is appended only
when given to improve backward compatibility.

When it is depending on the `zephyr,console` node, the backend
is defined as `log_backend_uart`.

When it is depending on the new `zephyr,log-uart` node, the
backend is defined as `log_backend_uart0`,
`log_backend_uart1`, and so on.

Updated the names of the internal variables to follow the same
naming convention.

Signed-off-by: Yong Cong Sin <ycsin@meta.com>