Unit tests are now separately linked

Rules for each test suites are now defined by each
module's makefile

* Excluded "HelloRaspberry.o" from product library
otherwise 'HelloRaspberry' will be the 'main'
method also for the test suites

* Renamed 'PROD_LIB_DIR' to 'LIB_DIR' because also
GTest lib will go into this folder for unit test
builds

* Cleaned up "gtest.mk" somewhat

* Introduced make variable
'GENERAL_TARGET_DEPENDENCIES' to collect all
dependencies that are common to all build variants

* Added 'ALL_UNIT_TESTS_COMMAND' which holds the
shell command for executing all unit test suites

* Added newlines at the end of most target recipes