configs/infra: fix initialization of ARM_TF_BUILD_FLAGS

ARM_TF_BUILD_FLAGS should start with an empty string and should be
appended to itself before adding additional macros to it. Fix this
up for all the infra reference platforms. In addition to this,
remove the unused ENABLE_SPM flag.

Signed-off-by: Thomas Abraham <thomas.abraham@arm.com>
Change-Id: Ia270b3c4191503c1d25466188f66b0ebf5e3126a