gdb: Include xz support by default and clean up PACKAGECONFIG

gdb: Include xz support by default and clean up PACKAGECONFIG

Firstly, just include xz support in all gdb configurations to simplify config.
Most systems would already have the shared library so this isn't a big problem
for a larger debugging tool.

The PACKAGECONFIG duplication is also confusing. The only PACKAGECONFIG which
needs special handking is the python one due to the differing modules needed
in the nativesdk case. Remove all the other duplicate entries which should work
through our usual class extension code.

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