fbt: lint fixes (#2008)

* lint: exclude "lib" subfolder from naming checks; fbt: moved LINT_SOURCES from literal strings to Dir() nodes
* lint: also exclude hidden directories

Co-authored-by: あく <alleteam@gmail.com>

firmware/SConscript

@@ -1,7 +1,7 @@
 Import("env")
 
 env.Append(
-    LINT_SOURCES=["firmware"],
+    LINT_SOURCES=[Dir(".")],
     SDK_HEADERS=[
         *env.GlobRecursive("*.h", "targets/furi_hal_include", "*_i.h"),
         *env.GlobRecursive("*.h", "targets/f${TARGET_HW}/furi_hal", "*_i.h"),