native: fix and improve term-valgrind flags

1. the wrong (non-existent) make target had been used for setting the flags conditionally,
2. the output is now more useful:
```
==5840== Conditional jump or move depends on uninitialised value(s)
==5840==    at 0x415A7DA: __mktime_internal (in /usr/lib32/libc-2.19.so)
==5840==    by 0x415A90D: mktime (in /usr/lib32/libc-2.19.so)
==5840==    by 0x80509EE: test_date_time (tests/unittests/tests-cbor/tests-cbor.c:543)
==5840==    by 0x805739E: TestCase_run (tests/unittests/embunit/embUnit/TestCase.c:58)
==5840==    by 0x8057319: TestCaller_run (tests/unittests/embunit/embUnit/TestCaller.c:54)
==5840==    by 0x805777E: TestRunner_runTest (tests/unittests/embunit/embUnit/TestRunner.c:99)
==5840==    by 0x8053F34: tests_cbor (tests/unittests/tests-cbor/tests-cbor.c:789)
==5840==    by 0x804C2DB: main (tests/unittests/main.c:30)
==5840==  Uninitialised value was created by a stack allocation
==5840==    at 0x805095B: test_date_time (tests/unittests/tests-cbor/tests-cbor.c:528)
```
and
```
==7246== Invalid read of size 4
==7246==    at 0x8049B9A: thread_measure_stack_free (core/thread.c:99)
==7246==    by 0x80511E4: thread_print_all (sys/ps/ps.c:69)
==7246==    by 0x804FE8E: _ps_handler (sys/shell/commands/sc_ps.c:25)
==7246==    by 0x804F59A: handle_input_line (sys/shell/shell.c:197)
==7246==    by 0x804F73C: shell_run (sys/shell/shell.c:262)
==7246==    by 0x804E542: main (examples/default/main.c:137)
==7246==  Location 0x805dbe8 is 0 bytes inside idle_stack[7016],
==7246==  a global variable declared at kernel_init.c:83
```
vs.
```
.==5913== Conditional jump or move depends on uninitialised value(s)
==5913==    at 0x415A7DA: __mktime_internal (in /usr/lib32/libc-2.19.so)
==5913==    by 0x415A90D: mktime (in /usr/lib32/libc-2.19.so)
==5913==    by 0x80509EE: test_date_time (tests-cbor.c:543)
==5913==    by 0x805739E: TestCase_run (TestCase.c:58)
==5913==    by 0x8057319: TestCaller_run (TestCaller.c:54)
==5913==    by 0x805777E: TestRunner_runTest (TestRunner.c:99)
==5913==    by 0x8053F34: tests_cbor (tests-cbor.c:789)
==5913==    by 0x804C2DB: main (main.c:30)
```
and
```
==7842== Invalid read of size 4
==7842==    at 0x8049B9A: thread_measure_stack_free (thread.c:99)
==7842==    by 0x80511E4: thread_print_all (ps.c:69)
==7842==    by 0x804FE8E: _ps_handler (sc_ps.c:25)
==7842==    by 0x804F59A: handle_input_line (shell.c:197)
==7842==    by 0x804F73C: shell_run (shell.c:262)
==7842==    by 0x804E542: main (main.c:137)
==7842==  Address 0x805dbe8 is 7016 bytes inside data symbol "idle_stack"
```