New --bbpath option and unecessary --rootfs checks

New --bbpath option and unecessary --rootfs checks

. Add the "--bbpath" option to pass an alternate busybox pathname instead of the one found from ${PATH}.
. Take this opportunity to add some formatting in the usage display
. As a try is done to pick rootfs from the config file and set it to ${path}/rootfs, it is unnecessary to make it mandatory

Signed-off-by: Rachid Koucha <rachid.koucha@gmail.com>