pstore/platform: Use backend name for console registration

If the pstore backend changes, there's no indication in the logs what
the console is (it always says "pstore"). Instead, pass through the
active backend's name. (Also adjust the selftest to match.)

Link: https://lore.kernel.org/lkml/20200510202436.63222-5-keescook@chromium.org/
Link: https://lore.kernel.org/lkml/20200526135429.GQ12456@shao2-debian
Signed-off-by: Kees Cook <keescook@chromium.org>

tools/testing/selftests/pstore/pstore_tests

@@ -10,7 +10,7 @@
 . ./common_tests
 
 prlog -n "Checking pstore console is registered ... "
-dmesg | grep -q "console \[pstore"
+dmesg | grep -Eq "console \[(pstore|${backend})"
 show_result $?
 
 prlog -n "Checking /dev/pmsg0 exists ... "