Add "legal-info" target to buildroot

This adds a manifest to the output which describes how to reproduce the
configuration and the licenses of the packages included.

See https://buildroot.org/downloads/manual/legal-notice.txt for more info.

Signed-off-by: Brian Cain <bcain@quicinc.com>

build-buildroot.sh

@@ -19,4 +19,5 @@ export FORCE_UNSAFE_CONFIGURE=1
 make -C buildroot/ O=../obj_buildroot/ qcom_dsp_qemu_defconfig
 cd obj_buildroot
 make -j
+make legal-info
 install -D ./images/* ${ARTIFACT_BASE}/${ARTIFACT_TAG}/