Rework the coverage build

This reworks it to avoid a need to special build type and adding the flags only
to the targets that need it (skipping testing wrappers which break with them).

It also updates the CodeCoverage module from the following URL:

https://github.com/bilke/cmake-modules/blob/master/CodeCoverage.cmake

Signed-off-by: Jakub Jelen <jjelen@redhat.com>
Reviewed-by: Andreas Schneider <asn@cryptomilk.org>

cmake/Modules/AddCMockaTest.cmake

@@ -116,5 +116,9 @@ function(ADD_CMOCKA_TEST _TARGET_NAME)
     add_test(${_TARGET_NAME}
         ${TARGET_SYSTEM_EMULATOR} ${_TARGET_NAME}
     )
+    if (WITH_COVERAGE)
+        include(CodeCoverage)
+        append_coverage_compiler_flags_to_target(${_TARGET_NAME})
+    endif (WITH_COVERAGE)
 
 endfunction (ADD_CMOCKA_TEST)