Re-introduce evaluation of ENABLE_PLAT_COMPAT build flag

Re-introduce evaluation of ENABLE_PLAT_COMPAT build flag

Commit #73c99d4eb377e0e25f7951be53087bf92e7b4b18 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