lualoader: Add nextboot support

config.parse now takes an extra callback that is invoked on the full text of
the config file. This callback dictates where we should actually try to
parse this file or not.

For nextboot, we use this to halt parsing if we see 'nextboot_enable="NO"'.
If we don't, parse it and write 'nextboot_enable="NO" ' to it. The same
caveat as with forth still applies- writing is only supported by UFS.