cmd/build: Upload files under rootfs directory

It is boring to write a Capstanfile's file section for apps contain a
large number of files, e.g. osv/apps/cassandra/Capstanfile.

A new option for Capstanfile is introduced: rootfs.
See Documentation/Capstanfile.md for more.

With this patch, we can put files which want to be injected into base
image under the rootfs directory, capstan build will inject them all.

Signed-off-by: Asias He <asias@cloudius-systems.com>