Fixing toolchain executable not found error for build.py

Commit 19d56fd40f removed the default file
paths for the toolchains. This was done under the assumption that the
top-level compile scripts were properly checking that the toolchain
executable was availble. The build.py script was doing this in the wrong
place. This commit rearranges the script a bit so the check is performed
properly.

tools/build.py

@@ -211,15 +211,17 @@ if __name__ == '__main__':
     successes = []
     skipped = []
 
-    # CPPCHECK code validation
-    if options.cppcheck_validation:
     for toolchain in toolchains:
         if not TOOLCHAIN_CLASSES[toolchain].check_executable():
             search_path = TOOLCHAIN_PATHS[toolchain] or "No path set"
             args_error(parser, "Could not find executable for %s.\n"
                                "Currently set search path: %s"
                        % (toolchain, search_path))
+
+    for toolchain in toolchains:
         for target in targets:
+            # CPPCHECK code validation
+            if options.cppcheck_validation:
                 try:
                     mcu = TARGET_MAP[target]
                     # CMSIS and MBED libs analysis
@@ -246,8 +248,6 @@ if __name__ == '__main__':
                     print e
             else:
                 # Build
-        for toolchain in toolchains:
-            for target in targets:
                 tt_id = "%s::%s" % (toolchain, target)
                 if toolchain not in TARGET_MAP[target].supported_toolchains:
                     # Log this later
@@ -299,6 +299,7 @@ if __name__ == '__main__':
                         failures.append(tt_id)
                         print e
 
+
     # Write summary of the builds
     print
     print "Completed in: (%.2f)s" % (time() - start)