Adjusting variables when building custom firmware

closure with simple optimizations gives

No errors in https://raw.githubusercontent.com/esprui­no/EspruinoDocs/master/devices/MPU6050_D­MP.js. Minifying 34114 bytes to 8589 bytes
No errors. Minified 14282 bytes to 13871 bytes.
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 2v06 (c) 2019 G.Williams
>Execution Interrupted
New interpreter error: LOW_MEMORY,MEMORY
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