configs: rename default-configs to configs and reorganise

In preparation for offering variation to our build configurations lets
move everything and rename it to default. Common included base configs
are also renamed.

During the cleanup the stale usb.mak and pci.mak references were
removed from MAINTAINERS.

Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Cc: Paolo Bonzini <pbonzini@redhat.com>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-Id: <20210707131744.26027-5-alex.bennee@linaro.org>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>