Makefile:Improve help and readme

Signed-off-by: Jari Helaakoski <tekkuli@gmail.com>
Makefile:Fix livesuit target

Fix for
./sunxi-bsp/sunxi-tools/fex2bin: not found
make: *** [livesuit] Error 127

Signed-off-by: Jari Helaakoski <tekkuli@gmail.com>