loader/multiboot: Do not add modules before successful download

loader/multiboot: Do not add modules before successful download

Multiboot modules that could not be read successfully, e.g. via network,
should not be added to the list of modules to forward to the operating
system that is to be booted subsequently.

This patch is necessary because even if a grub.cfg checks whether or not
a module was successfully downloaded, it is futile to retry a failed
download as the corrupted module will be forwarded either way.

Signed-off-by: Valentin Gehrke <valentin.gehrke@kernkonzept.com>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>