$roboto-font-path: 'fonts' !default;

@mixin roboto-font($type, $weight, $style: normal) {
    @font-face {
        font-family: 'Roboto';
        src: url('#{$roboto-font-path}/Roboto-#{$type}.eot');
        src: url('#{$roboto-font-path}/Roboto-#{$type}.eot?#iefix') format('embedded-opentype'),
             url('#{$roboto-font-path}/Roboto-#{$type}.woff2') format('woff2'),
             url('#{$roboto-font-path}/Roboto-#{$type}.woff') format('woff'),
             url('#{$roboto-font-path}/Roboto-#{$type}.ttf') format('truetype'),
             url('#{$roboto-font-path}/Roboto-#{$type}.svg#Roboto') format('svg');
        font-weight: $weight;
        font-style: $style;
    }
}

@mixin roboto-font-pair($type, $weight) {
    @include roboto-font($type, $weight);
    @include roboto-font(#{$type}Italic, $weight, italic);
}

@include roboto-font-pair('Thin', 100);
@include roboto-font-pair('Light', 300);
@include roboto-font-pair('Regular', 400);
@include roboto-font-pair('Medium', 500);
@include roboto-font-pair('Bold', 700);
@include roboto-font-pair('Black', 900);

$roboto-font-path: 'fonts' !default;

@mixin roboto-font($type, $weight, $style: normal) {
    @font-face {
        font-family: 'Roboto-#{$type}';
        src: url('#{$roboto-font-path}/Roboto-#{$type}.eot');
        src: url('#{$roboto-font-path}/Roboto-#{$type}.eot?#iefix') format('embedded-opentype'),
             url('#{$roboto-font-path}/Roboto-#{$type}.woff2') format('woff2'),
             url('#{$roboto-font-path}/Roboto-#{$type}.woff') format('woff'),
             url('#{$roboto-font-path}/Roboto-#{$type}.ttf') format('truetype'),
             url('#{$roboto-font-path}/Roboto-#{$type}.svg#Roboto') format('svg');
        font-weight: $weight;
        font-style: $style;
    }
}

@mixin roboto-font-pair($type, $weight) {
    @include roboto-font($type, $weight);
    @include roboto-font(#{$type}Italic, $weight, italic);
}

@include roboto-font-pair('Thin', 100);
@include roboto-font-pair('Light', 300);
@include roboto-font-pair('Regular', 400);
@include roboto-font-pair('Medium', 500);
@include roboto-font-pair('Bold', 700);
@include roboto-font-pair('Black', 900);

Use type variable also in Font-Family name.
Use  type variable also in Font-Family name.

Currently all variants of the font get the same name "Roboto" that makes no sense.