modules: Add support for resolving variables in manifest rules

Manifests should contain paths in the same file system in which the
image is built. Currently manifests assumed that the module will be
downloaded/copied into build/*/module/ and that the CWD is inside
build/*. This is not always the case now that we have "direct-dir"
module type, which is is not copied.

Example use:

    /usr/x:${MODULE_DIR}/x

This patch adds MODULE_DIR variable, which is replaced with the path
to the module in local file system. Depending on the module type this
can be either direct path or a path inside build/*/module/.

Signed-off-by: Tomasz Grabiec <tgrabiec@cloudius-systems.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>