The evtimer_msg test is expanded to also delete entries.

Furthermore the messages that are printed should now show
numbers that are very close (if not equal). Something like
this:
At    740 ms received msg 0: "RIOT-OS#2 supposed to be 740"
At   1081 ms received msg 1: "#0 supposed to be 1081"
At   1581 ms received msg 2: "#1 supposed to be 1581"
At   4035 ms received msg 3: "RIOT-OS#3 supposed to be 4035"

The function evtimer_print is also called to show the
intermediate status of evtimer entries.

Add function calls to use PSK

The test randomly fails on `native` due to timers being not accurate but
it cannot be otherwise. So better disable it than raising fake errors.

    main(): This is RIOT! (Version: buildtest)
    Testing generic evtimer
    This should list 2 items
    ev #1 offset=1000
    ev RIOT-OS#2 offset=500
    This should list 4 items
    ev #1 offset=659
    ev RIOT-OS#2 offset=341
    ev RIOT-OS#3 offset=500
    ev RIOT-OS#4 offset=2454
    Are the reception times of all 4 msgs close to the supposed values?
    At    662 ms received msg 0: "RIOT-OS#2 supposed to be 659"
    At   1009 ms received msg 1: "#0 supposed to be 1000"
    At   1511 ms received msg 2: "#1 supposed to be 1500"

    Traceback (most recent call last):
      File "/tmp/dwq.0.3125418833043728/ef3af88c4b3615788b164464a437df5c/tests/evtimer_msg/tests/01-run.py", line 33, in <module>
        sys.exit(run(testfunc))
      File "/tmp/dwq.0.3125418833043728/ef3af88c4b3615788b164464a437df5c/dist/pythonlibs/testrunner/__init__.py", line 29, in run
        testfunc(child)
      File "/tmp/dwq.0.3125418833043728/ef3af88c4b3615788b164464a437df5c/tests/evtimer_msg/tests/01-run.py", line 26, in testfunc
        assert(actual in range(expected - ACCEPTED_ERROR, expected + ACCEPTED_ERROR))
    AssertionError

The ROM size is encoded in the part number of the Atmel SAM chips.
RAM size is not encoded directly, so get it by parsing the chip's vendor file.

The file remains in the page cache for the compiler to use, so the overhead
should be minimal:

on master:

  Benchmark #1: make BOARD=samr21-xpro -j
    Time (mean ± σ):     527.9 ms ±   4.9 ms    [User: 503.1 ms, System: 69.6 ms]
    Range (min … max):   519.7 ms … 537.2 ms    10 runs

with this patch:

  Benchmark #1: make BOARD=samr21-xpro -j
    Time (mean ± σ):     535.6 ms ±   4.0 ms    [User: 507.6 ms, System: 75.1 ms]
    Range (min … max):   530.6 ms … 542.0 ms    10 runs

Coverty scan found this:

> CID 298279 (#1 of 1): Out-of-bounds read (OVERRUN)
> 21. overrun-local: Overrunning array of 16 bytes at byte offset 64 by dereferencing pointer

The original intention was probably to advance the destination pointer by 4 bytes, not
4 * the destination type size.

Coverty scan found this:

> CID 298295 (#1 of 1): Operands don't affect result (CONSTANT_EXPRESSION_RESULT) result_independent_of_operands:
> (ipv6_hdr_get_fl(ipv6_hdr) & 255) >> 8 is 0 regardless of the values of its operands.

Looking at the code, this appears to be a copy & paste error from the previous line.