Bug 1054331 - Build valgrind only when B2G_VALGRIND=1.

The b2g.mk script only checked if B2G_VALGRIND variable was empty, so
even when it was set to 0, valgrind was built.

Now we check if the B2G_VALGRIND value is 1, so if it empty valgrind
will not build.
Merge pull request #179 from walac/master

Bug 1054331 - Build valgrind only when B2G_VALGRIND=1. r=dhylands