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.

With this patch, we can put files which want to be injected into osv
image under the ROOTFS directory. capstan build will inject them all.

Signed-off-by: Asias He <asias@cloudius-systems.com>