recipes/classes: Drop prepend/append usage with PACKAGESPLITFUNCS

As far as I can tell, none of these uses of PACKAGESPLITFUNCS need append/prepend
operators, the standard += and =+ can work just fine. Since OE-Core is copied a lot,
use the preferred syntax which is also simpler to parse and change.

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>