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Following changes are done to make DT pre-processing similar to that of
U-boot/Linux kernel.

1. Creating seperate CPPFLAGS for DT preprocessing so that compiler
options specific to it can be accommodated.
e.g: "-undef" compiler option avoids replacing "linux" string(used in
device trees) with "1" as "linux" is a pre-defined macro in gnu99
standard.

2. Replace CPP with PP for DT pre-processing, as CPP in U-boot/Linux is
exported as "${CROSS_COMPILE}gcc -E" while in TF-A it is exported as
"${CROSS_COMPILE}cpp".

Change-Id: If4c61a249d51614d9f53ae30b602036d50c02349
Signed-off-by: Manish Pandey <manish.pandey2@arm.com>