This addresses the issue where building tests via test.py always triggered
a clean build. This is because the mbed_config.h file was being deleted from
the shared OS build to ensure that the correct config was always being
used. However, this contanstly triggered a rebuild of the OS since the
config file was not present.
Due to the shared build, having multiple app configurations that could
override the OS settings is not possible. For this reason, we now ignore
app config files unless explicitly set via the command line option
'--app-config'. Though there will now be two mbed_config.h files in the
include path of the build, it shouldn't matter since the contents will be
the same.