style: add cstyle target to Makefile + clang-format config, don't enforce for now

.clang-format

@@ -0,0 +1,3 @@
+---
+BasedOnStyle:  Google
+IndentWidth:   4

Makefile

@@ -73,6 +73,8 @@ test_emu: ## run selected device tests from python-trezor
 pylint: ## run pylint on application sources and tests
 	pylint -E $(shell find src tests -name *.py)
 
+## style commands:
+
 style: ## run code style check on application sources and tests
 	flake8 $(shell find src -name *.py)
 	isort --check-only $(shell find src -name *.py ! -path 'src/trezor/messages/*')
@@ -84,6 +86,9 @@ isort:
 black:
 	black $(shell find src -name *.py ! -path 'src/trezor/messages/*')
 
+cstyle: ## run code style check on low-level C code
+	./tools/clang-format-check $(shell find embed -type f -name *.[ch])
+
 ## build commands:
 
 build: build_boardloader build_bootloader build_firmware build_prodtest build_unix ## build all

tools/clang-format-check

@@ -0,0 +1,10 @@
+#!/bin/sh
+error=0
+for f in $*; do
+    changes=$(clang-format -output-replacements-xml "$f" | grep -c '^<replacement ')
+    if test "$changes" -gt 0; then
+        echo "$f ... $changes changes needed"
+        error=1
+    fi
+done
+exit $error