Add config option to disable low battery check

We have encountered a device where the connection from the PMIC's ADMUX pin to the MCUs ADC is severed and thus the low battery check would trigger immediately.

For making such devices still usable, add a config option to disable the low battery check entirely.

Fixes: #227 (closed)

Documentation/card10-cfg.rst

@@ -62,4 +62,6 @@ Option name        Type       Description
 ``bsec_debug``     Boolean    Turn on debug output of the BSEC system. Prints each meaurement on the console.
 ------------------ ---------- -----------
 ``bsec_offset``    Integer    Temperature offset in .1 K. Example: Set to `-14` if temperature reads 1.4 "C" to high. Default: -2.2 K (appropriate for a card10 without a case, connected to USB and with BLE active in vertical orientation).
+------------------ ---------- -----------
+``battery_check``  Boolean    Whether the low battery check should be enabled (default ``true``).  **Warning**: Do not use this unless you know what you're doing.  This option is only meant to be used on devices with a broken PMIC ADMUX connection.
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