fbt: added support for FBT_NO_SYNC environment variable to skip submodule update at start (#1363)

fbt.cmd

@@ -2,8 +2,13 @@
 
 set SCONS_EP=%~dp0\lib\scons\scripts\scons.py
 
-if exist ".git" (
-	git submodule update --init
+if [%FBT_NO_SYNC%] == [] (
+	if exist ".git" (
+		git submodule update --init
+	) else (
+		echo Not in a git repo, please clone with git clone --recursive
+		exit /b 1
+	)
 )
 
 set "SCONS_DEFAULT_FLAGS=-Q --warn=target-not-built"