Rework common.mk

The code added by commits d6536da6 ("Commonlize xtest compilation
among all platforms") and 070d9559 ("common build: optee
components") introduces race conditions that break parallel build (make
-jX). This is due to manually launching a make sub-process to run the
-common targets, and can be fixed by just declaring a dependency
instead. Specific flags are appended to common flags, rather than
passed as arguments to a new "make" sub-process.

So, basically:

 foo-common:
 	$(MAKE) -C foo_dir [A=a B=B]

 foo:
 	$(MAKE) foo-common [C=c]

is turned into:

 FOO_COMMON_FLAGS ?= A=a B=b
 foo-common:
 	$(MAKE) -C foo_dir $(FOO_COMMON_FLAGS)

 FOO_COMMON_FLAGS += C=c
 foo: foo-common

At the same time this allows foo: to override flags, for instance
set A=aa in the above example (prior to this, A=a would have had a
higher priority).

Signed-off-by: Jerome Forissier <jerome.forissier@linaro.org>
Signed-off-by: Pascal Brand <pascal.brand@st.com>
Reviewed-by: Jerome Forissier <jerome.forissier@linaro.org>