Build: Introduce TARGET_PLATFORM_PATH

The TARGET_PLATFORM_PATH refers to the root directory of
target platform to build, for example
"platform/ext/target/arm/mps2/an521".

With the TARGET_PLATFORM_PATH, lots of references to this path with long
paths can be avoided.

Change-Id: Iaa536cac8048de803fc4a002641e066563679722
Signed-off-by: Kevin Peng <kevin.peng@arm.com>
9 files changed