mod/fwk: Add conditional code coverage support for Unit Tests

This patch introduces a new ENABLE_COVERAGE flag to the Makefile
wrapper, allowing for conditional generation of code coverage reports
for `fwk_test` and `mod_test` targets

Signed-off-by: Leandro Belli <leandro.belli@arm.com>
Change-Id: Ib4fae6a8ab739c5b993daef7893d606634491d46