jffs2-image: add variable for customization of mkfs.jffs2 options
jffs2-image: add variable for customization of mkfs.jffs2 options

sknsean/core,sknsean/core,diorcety/oe-lite-core,sknsean/core,diorcety/oe-lite-core,hundeboll/core,hundeboll/core,hundeboll/core,sknsean/core,sknsean/core,hundeboll/core