dtlib: add lineno/filename to string representation of a DT

Output lineno/filename as comments in the string representation of a DT
to help with debugging DT issues. Also, remove the added comments when
comparing the string representation of a DT to a reference string in the
DT testsuite.

Signed-off-by: Benjamin Cabé <benjamin@zephyrproject.org>
Signed-off-by: Luca Burelli <l.burelli@arduino.cc>