gettext: Improve USE_NLS handling for nativesdk/crosssdk/cross-canadian
The gettext handling of USE_NLS has become a bit tricky to understand, or
alter from the SDK context. This patch introduces a SDKUSE_NLS which can
be set to configure a given SDK/ADT to use NLS or not. This is independent
of the target system NLS usage.
The code in gettext.bbclass is therefore simplified and the classes
themselves now set USE_NLS to appropriate values. No NLS is used
for native, cross and crosssdk since it is never used there and
would just increase build time.
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
