fwbank: Use a private cache for system layout variables used in the ubus object
This change reduces the time for the 'dump' method to execute.
Output of `time ubus call fwbank dump`:
Before change
real 0m 0.36s
user 0m 0.00s
sys 0m 0.00s
After change, cache empty:
real 0m 0.38s
user 0m 0.00s
sys 0m 0.00s
After change, cache filled with bank info and system layout variable data:
real 0m 0.12s
user 0m 0.00s
sys 0m 0.00s