file paths: add PX4_STORAGEDIR & use it where appropriate

src/modules/dataman/dataman.cpp

src/modules/logger/logger.cpp

src/modules/logger/logger.h

src/modules/mavlink/mavlink_ftp.cpp

src/modules/mavlink/mavlink_log_handler.cpp

src/modules/mavlink/mavlink_receiver.cpp

src/modules/mavlink/mavlink_tests/mavlink_ftp_test.cpp

src/modules/navigator/geofence.h

.../position_estimator_inav/position_estimator_inav_main.cpp

src/modules/uORB/uORBManager.cpp

src/modules/uORB/uORB_tests/uORBTest_UnitTest.cpp

src/platforms/px4_defines.h

src/systemcmds/sd_bench/sd_bench.c

src/systemcmds/tests/test_file.c

src/systemcmds/tests/test_file2.c

src/systemcmds/tests/test_mount.c

src/systemcmds/tests/test_parameters.cpp