net: When checking prerequisites, consider boot_file_name

For net_boot_common, we allow the serverip to be specified as part of
the boot file name. For net commands that require serverip, include that
source as a valid specification of serverip.

Signed-off-by: Joe Hershberger <joe.hershberger@ni.com>
Reviewed-by: Alexander Graf <agraf@suse.de>