Makefile.target: support per-target coverage reports

Add support for generating a single targets coverage report. Execute:

  make coverage-report

In the target build directory. This coverage report only cares about
target specific blobs so only searches the target build subdirectory.

Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>