Update Bootstrap.mak (#2645)

MinGW: (w64devkit)
~ $  mingw32-make -f Bootstrap.mak mingw CONFIG=x64
. . .
make -C build/bootstrap config=x64_win32
make[1]: Entering directory 'D:/CPP/SOURCE/Premake/premake-core-5.0.0-beta8/build/bootstrap'
Makefile:34: *** "invalid configuration x64_win32".  Stop.
make[1]: Leaving directory 'D:/CPP/SOURCE/Premake/premake-core-5.0.0-beta8/build/bootstrap'
make: *** [Bootstrap.mak:112: mingw] Error 2

replacing CONFIG with PLATFORM  builds succesfully.

NOTE:
  CONFIG : 
    release (default)
    debug
  PLATFORM: 
    x86 (default)
    x64

Author: 0hip

Bootstrap.mak

@@ -72,7 +72,7 @@ none:
 	@echo "Please do"
 	@echo "   nmake -f Bootstrap.mak windows"
 	@echo "or"
-	@echo "   CC=mingw32-gcc mingw32-make -f Bootstrap.mak mingw CONFIG=x64"
+	@echo "   CC=mingw32-gcc mingw32-make -f Bootstrap.mak mingw PLATFORM=x64"
 	@echo "or"
 	@echo "   make -f Bootstrap.mak HOST_PLATFORM"
 	@echo "where HOST_PLATFORM is one of these:"
@@ -86,7 +86,7 @@ clean:
 	@echo "Please run the same command used for building by adding a '-clean' suffix to the target name."
 	@echo "   nmake -f Bootstrap.mak windows-clean"
 	@echo "or"
-	@echo "   CC=mingw32-gcc mingw32-make -f Bootstrap.mak mingw-clean CONFIG=x64"
+	@echo "   CC=mingw32-gcc mingw32-make -f Bootstrap.mak mingw-clean PLATFORM=x64"
 	@echo "or"
 	@echo "   make -f Bootstrap.mak HOST_PLATFORM-clean"
 	@echo "where HOST_PLATFORM is one of these:"