dtoc: Allow DTC environment variable to provide path to dtc

The system device-tree compiler may not be new enough to run the tests we
use in U-Boot (e.g. with binman). Allow use of a DTC environment variable
to point to the correct dtc. If not defined, the dtc on the default PATH
is used.

Signed-off-by: Simon Glass <sjg@chromium.org>

tools/binman/README

@@ -536,6 +536,10 @@ entry contents.
 Most of the time such essoteric behaviour is not needed, but it can be
 essential for complex images.
 
+If you need to specify a particular device-tree compiler to use, you can define
+the DTC environment variable. This can be useful when the system dtc is too
+old.
+
 
 History / Credits
 -----------------

tools/dtoc/fdt_util.py

@@ -79,7 +79,8 @@ def EnsureCompiled(fname):
             '-W', 'no-unit_address_vs_reg']
     args.extend(search_list)
     args.append(dts_input)
-    command.Run('dtc', *args)
+    dtc = os.environ.get('DTC') or 'dtc'
+    command.Run(dtc, *args)
     return dtb_output
 
 def GetInt(node, propname, default=None):