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This replaces problematic data pass in Lua with files saved on disk.
Original problem is that Lua in some cases like closures duplicates
environment and that means also all data. If this happens few times
(from measurements it seems like two or three times in code segment we
are dropping in this) then we can have few times bigger memory
requirements.

This also temporally disables test test-sys-pkgupdate-plan-unapproved as
it does not expect additional files. This is going to be solved in
following commit.

We do change directory and it makes sense to use absolute path either
way so let's go for it.

This also moves some configuration variables from backend to separate
syscnf module.
Original design where hard coded in values were used as defaults is not
replaced with design where set function is required to be called before
anything is done. Commonly that is suppose to be done in C before we
start using other Lua modules.
Thanks to new options --model and --board we can now set target even
outside of the router environment without requiring /tmp tainting. It's
now also not possible to run updater's executables on non-openwrt
platform without specifying those two options.

Check if error output contains both error messages when multiple errors
happens.

Provide an error handler. Also, store the auto-loaded "modules" to
global space.