Allow multiple DEVICE_MATRIX_FILE.

Allow to merge multiple items in DEVICE_MATRIX_FILE.
Test: build device matrix
Bug: 130714844

Change-Id: I81783e7525fb4933768ef5fbb1ea4cf01d7c50a5
Merged-In: I81783e7525fb4933768ef5fbb1ea4cf01d7c50a5
5 files changed