layer.conf: Filter docs dependencies for efficiency
Where a recipe has depends on native docs tools, in most cases
we don't need recipes that depend on that recipe to also install
these things into the sysroot. We can rely on recipes wanting these
tools to have direct dependencies instead.
This massively reduced dependency creep in simple recipes (e.g. an
allarch one) and reduced the size of builds with the api-documentation
feature substancially.
gperf-native is also included since that would normally have a direct
dependency in a recipe which needs it too.
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
