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As an initial stage of making Trusted Firmware build environment more
portable, we remove most uses of the $(shell ) function and replace them
with more portable make function based solutions.

Note that the setting of BUILD_STRING still uses $(shell ) since it's
not possible to reimplement this as a make function. Avoiding invocation
of this on incompatible host platforms will be implemented separately.

Change-Id: I768e2f9a265c78814a4adf2edee4cc46cda0f5b8