Update bionic coverage docs.

These docs refer to the old coverage workflow from the Android.mk
build.

AFAIK there's no way to get coverage for the host tests, so that part
was removed.

Test: followed the docs, got coverage data
Bug: None
Change-Id: Ibef5ae2d354296a77b752d3b7c9c48cf7a34ef98

README.md

@@ -271,39 +271,11 @@ the host's glibc.
 
     $ ./tests/run-on-host.sh glibc
 
-
 ## Gathering test coverage
 
-For either host or target coverage, you must first:
-
- * `$ export NATIVE_COVERAGE=true`
-     * Note that the build system is ignorant to this flag being toggled, i.e. if
-       you change this flag, you will have to manually rebuild bionic.
- * Set `bionic_coverage=true` in `libc/Android.mk` and `libm/Android.mk`.
-
-### Coverage from device tests
-
-    $ mma
-    $ adb sync
-    $ adb shell \
-        GCOV_PREFIX=/data/local/tmp/gcov \
-        GCOV_PREFIX_STRIP=`echo $ANDROID_BUILD_TOP | grep -o / | wc -l` \
-        /data/nativetest/bionic-unit-tests/bionic-unit-tests
-    $ acov
-
-`acov` will pull all coverage information from the device, push it to the right
-directories, run `lcov`, and open the coverage report in your browser.
-
-### Coverage from host tests
-
-First, build and run the host tests as usual (see above).
-
-    $ croot
-    $ lcov -c -d $ANDROID_PRODUCT_OUT -o coverage.info
-    $ genhtml -o covreport coverage.info # or lcov --list coverage.info
-
-The coverage report is now available at `covreport/index.html`.
-
+To get test coverage for bionic, use `//bionic/build/coverage.sh`. Before
+running, follow the instructions at the top of the file to rebuild bionic with
+coverage instrumentation.
 
 ## Attaching GDB to the tests