Update 01-rootfs-recipe.md

hello_2.8-4_armhf.deb doesn't exists (there), replaced by hello_2.8-2_armhf.deb (hello_2.9-1_armhf.deb should work)
small update

apt-get update forgotten
path for Udoo kernel config updated
typo correction
Merge pull request #7 from mmeinert/master

multiple small updates from @mmeinert