Add description of this tree

See buildroot/docs/manual/customize-outside-br.adoc

Signed-off-by: Jean-Philippe Brucker <jean-philippe@linaro.org>

README

+This external buildroot tree contains packages and modifications to run an
+attestation demo for Arm CCA.
+
+Use it while configuring buildroot:
+
+  make BR2_EXTERNAL=path/to/this/dir/ cca_defconfig
+

external.desc

+name: LINARO_CCA
+desc: Tools for CCA

external.mk

+include $(sort $(wildcard $(BR2_EXTERNAL_LINARO_CCA_PATH)/package/*/*.mk))