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Commit #73c99d4e had refactored the top level
Makefile. This commit also broke platform ports that still rely on an enabled
ENABLE_PLAT_COMPAT build option since the evaluation of this option was also
accidentally removed from the Makefile.

This patch fixes this break by re-introducing the necessary support to ensure
that this build option is enabled by default if a platform port does not disable
it explicitly.

Fixes ARM-software/tf-issues#332

Change-Id: I2217595d2e0bccae7de98cc6c0ea448b5bf3dae2