system: support passing "options" to the "sysupgrade" ubus method

system: support passing "options" to the "sysupgrade" ubus method

Object passed as "options" gets translated into environment variables
UPGRADE_OPT_*. E.g.
"options": { "foo": 5 }
will result in setting UPGRADE_OPT_FOO=5.

This allows stage2 sysupgrade to get options explicitly. So far it was
guessing what to do by checking for existence of some files (e.g.
sysupgrade.tgz).

Signed-off-by: Rafał Miłecki <rafal@milecki.pl>