common: Remove *_CLEAN_COMMON_FLAGS dependency on *_COMMON_FLAGS

Fixes https://github.com/OP-TEE/build/issues/28

.

In common.mk *_CLEAN_COMMON_FLAGS ?= *_COMMON_FLAGS but in the
platform makefiles (e.g. hikey.mk) they're defined as
such:
*_COMMON_FLAGS += x y z
*_CLEAN_COMMON_FLAGS += x

When expanded during a build, *_CLEAN_COMMON_FLAGS becomes
"x y z x". Variables can be repeated, which may be nothing more
than a cosmetic issue, but if any of those variables are make
targets, then 'make clean' will build them before cleaning.

To fix this, we remove the dependency of *_CLEAN_COMMON_FLAGS on
*_COMMON_FLAGS.

Signed-off-by: Victor Chong <victor.chong@linaro.org>
Tested-by: Victor Chong <victor.chong@linaro.org> (hikey)