libatf: pass atf root directory to build libatf

Vitor Sato Eschholzrequested to merge
vsatoes/atf-root into libatf

libatf need to have access to arm-trusted-firmware headers. Add rootATF build option to pass the root directory of the arm-trusted-firmware project.

Signed-off-by: Vitor Sato Eschholz vsatoes@baylibre.com

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