move config to a subdir

This pattern is used in other places, e.g.,
https://github.com/openbmc/phosphor-logging

In this way, we can cleanly add the bmcweb_config.h into include
directory. Otherwise

move config to a subdir

This pattern is used in other places, e.g.,
https://github.com/openbmc/phosphor-logging

In this way, we can cleanly add the bmcweb_config.h into include
directory. Otherwise, any subdir we have in this project will need to
include the root.

Tested: it builds.

Signed-off-by: Nan Zhou <nanzhoumails@gmail.com>
Change-Id: I882e51f3acb256a881c9474f6e4d4e19fea4a413

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