この漢字くらい読めろよ！の「読めろ」ですね。hmm we don't say 読められる but we seem to have derivative words like 読めろ、書けろ. But we can also say このぐらい 歌えろよ or 歌えれよ. What do you explain this?
That's my question to Amatuka-san, when he says There is no imperative form for potential verbs. [e.g. You cant say "You! Be able to read!" ]
このくらい_読め_てよ or 書け_てよ are not potential but imperative form. 読め_ろ、書け_ろでわかれるんじゃない？ろ may be this?（間投助）〔上代語〕文の終わり、または文中の連用修飾語に付いて、感動の意を添えて聞き手に働きかけるのに用いられる。from 大辞林. what do you think?
ろ I think is a particle which attach to the imperative form of verb to show it is an order. It used to be a dialect of Kanto area in the old days, but became popular even in Kyoto during the Middle Ages (12th Century to 16th Century in Japan).
That means 読め(+ろ)、書け(+ろ) are imperative form of 読む、書く and are not of potential verbs 読める、書ける. With this, you can exaplin 歌え+ろよand 歌えれ+よ(imperative form for potentioal verb, too. The former of dictional form is 歌う and the latter is 歌える（可能動詞）. どうでしょうか？
I give up, Miki-san. 読めろ and 書けろ are not proper Japanese but they are the products of the younger generation to begin with. I have no incentive or desire to defend them.
I don't know if they are so called correct Japanese or not. Let's leave them.
Formed from verb potential -masu base + kkonai Vpm = Verb potential -masu base. (貰える→貰えます→貰え)
→ Potential form : Ex. kaku (write) → kakeru (able to write), yomu (read) → yomeru (able to read) and such forms. There is no imperative form for potential verbs. [e.g. You cant say "You! Be able to read!" ;-) ]