Print error on missing device target_files archive

Check the newly-created DEVICE_TARGET_FILES_ARCHIVE variable instead of
checking the DEVICE_ARCHIVE variable twice.

Test: run build_mixed with only the *-img-* archive in the device directory
Merged-In: I59d9b8f77cac1dcdeb2c88cdf37bc8dfbd1111ca
Change-Id: I59d9b8f77cac1dcdeb2c88cdf37bc8dfbd1111ca
(cherry picked from commit c1474312b3babf7470508382a41011aaa94a98b7)
1 file changed