Directory path : D:GDAL-TEST
001_mean.tif
002_mean.tif
003_mean.tif
004_mean.tif
So on....

gdal_translate -of "ENVI" D:GDAL-TEST01_mean.tif D:GDAL-TEST01_mean.bil

for %i in (*.tif) do gdal_translate -of "ENVI" %i %i.bil

for %%i in (*.tif) do gdal_translate -of "ENVI" %%i %%i.bil