add CUSTOM_MTD_SIZE for alien firmware (fixes r9838)
 * why  has 7570_IAD 244 and not 246 blocks, btw 122 and not 119 like other -> r9838 ?

git-svn-id: file:///var/svn/freetz/trunk@9842 149334a1-2f27-0410-a3b9-fc62619ac1e6