doc: cmake: Do not use USES_TERMINAL on latex/pdf targets

The following error appeared on documentation builds when enabling the
USES_TERMINAL flag:

  USES_TERMINAL may not be specified without any COMMAND

This is because targets without a command should not be using the flag
at all, so remove its usage from those. Instead, use it in the
add_custom_command() that actualy invokes sphinx.

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