gettext.bbclass, bitbake.conf: Append nls options to EXTRA_OECONF instead of +=
Some recipes do not defined EXTRA_OECONF in such cases += drops
the --enable|--disable-nls options. In another case where recipe
defines EXTRA_OECONF instead of adding/appending to it then
--enable|--disable-nls options are lost from EXTRA_OECONF
We define EXTRA_OECONF = "" in bitbake.conf so the variable exists
always.
We use _append instead of += so the option is added at very end
and not lost.
We only return empty gettext dependencies if its a target recipe
in case when USE_NLS is not set because the native/cross/nativesdk recipes still
need the gettext dependencies
