Platforms/ARM/Juno: add PLATFORM_FLAGS rather than override them
Platforms/ARM/Juno: add PLATFORM_FLAGS rather than override them

The ArmJuno.dsc [BuildOptions] section adds some include paths to the
compiler flags, by overriding the PLATFORM_FLAGS build variable of all
build types, toolchains and architectures, using an == assignment.

Since some of PLATFORM_FLAGS variants may contain compiler switches that
we may want to keep, using = to add rather than replace is more appropriate
here.

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Ard Biesheuvel <66d3c5fdaeea7ff1f996ad04f2c45e08ab38e2f5@linaro.org>
Reviewed-by: Leif Lindholm <a613079f0455438b29e28a9c5b9eb3779a81a33a@linaro.org>

hisilicon/OpenPlatformPkg,jlinton/OpenPlatformPkg,hisilicon/OpenPlatformPkg,jlinton/OpenPlatformPkg