Add rule for building coverage summary using lcov.

Also modified a few tests to be more compatible with coverage information,
so that they use the same pb_encode/decode.c instead of making a copy.

tests/Makefile

@@ -4,3 +4,18 @@ all:
 clean:
 	scons -c
 
+coverage:
+	rm -rf build coverage
+
+	# LCOV does not like the newer gcov format
+	scons CC=gcc-4.6 CXX=gcc-4.6
+
+	# We are only interested in pb_encode.o and pb_decode.o
+	find build -name '*.gcda' -and \! \( -name '*pb_encode*' -or -name '*pb_decode*' \) -exec rm '{}' \;
+
+	# Collect the data
+	mkdir build/coverage
+	lcov --base-directory . --directory build/ --gcov-tool gcov-4.6 -c -o build/coverage/nanopb.info
+
+	# Generate HTML
+	genhtml -o build/coverage build/coverage/nanopb.info