fix OTA update on Kobo, close #824
fix OTA update on Kobo, close #824

lgeek/koreader,ashhher3/koreader,chihyang/koreader,NickSavage/koreader,chrox/koreader,pazos/koreader,mihailim/koreader,koreader/koreader,apletnev/koreader,frankyifei/koreader,Frenzie/koreader,houqp/koreader,poire-z/koreader,robert00s/koreader,poire-z/koreader,noname007/koreader,NiLuJe/koreader,Markismus/koreader,ashang/koreader,NiLuJe/koreader,Frenzie/koreader,mwoz123/koreader,koreader/koreader,Hzj-jie/koreader