It saves in a system temp folder with Windows. When I get back to my dev machine I will look. The value ESP8266_WEMOS_D1MINI is in a boards.txt file, so it looks like this is data for a script to build the makefiles. If ESP8266_WEMOS_D1MINI is supposed to be the actual text, then the command line needs to include quotes (not a command line expert on how to do this, but the first quote just states this is the value for the define (maybe single quote works for this), so ' then " then string, then ", then '? '"ESP8266_WEMOS_D1MINI"'