populate_ext_sdk: Merge auto.conf into local.conf

populate_ext_sdk: Merge auto.conf into local.conf

auto.conf is included before local.conf. Instead of keeping them separate, merge
them into the extsdk local.conf. As it happens we can do this quite neatly, more
neatly than the current code IMO and it makes the configuration easier for the end
user to understand too.

This means auto.conf is then available for the testsdk code to use for testing
purposes.

(From OE-Core rev: fb625e555707478d5b4931c6bc83ab06b17a8ca9)

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