devicetree.bbclass: Combine stderr into stdout to see actual dtc error
Previously the subprocess command to run dtc was not properly displaying
the error on console. Combining stderr into stdout for the dtc subprocess
so the actual error can be seen on console without having to open the
do_compile log.
For example, previously on a dtc error, just the following stack trace
and dtc command was being shown on console:
File: 'exec_python_func() autogenerated', lineno: 2, function: <module>
0001:
*** 0002:devicetree_do_compile(d)
0003:
File:
function: devicetree_do_compile
0127: if not(os.path.isfile(dtspath)) or
not(dts.endswith(".dts") or devicetree_source_is_overlay(dtspath)):
0128: continue # skip non-.dts files and non-overlay
files
0129: except:
0130: continue # skip if can't determine if overlay
*** 0131: devicetree_compile(dtspath, includes, d)
