Bug 837846 - Fix MOZ_MAKE_FLAGS handling for client.mk. r=mshal

After bug 762358 mk_add_options MOZ_MAKE_FLAGS was simply ignored in client.mk
processing. At the same time, mach environment was expecting a list of options
while the mozconfig reader returned a single string, so straighten this up at
the same time.
Bug 1071438 - Make client.mk detect python like configure and mach do. r=mshal