Add BAS GATT reporting

[Nicell]

This adds BAS GATT reporting using any device binding with the label BATTERY. Closes #47.

Notes:

• bt_gatt_bas_set_battery_level has been renamed in Zephyr 2.4.0 -> Upgrade to Zephyr 2.4.0 #223
• zmk_battery_update should eventually raise a battery event instead of directly posting to the BAS GATT
• If the board doesn't have a battery reporting device, init will fail and none of the timers will run. Should this be behind a Kconfig value instead?
• I'm thinking that the update function should only update if the battery value has changed since last time. This would for example save from an unnecessary ePaper display refresh in the future.

[petejohanson]

Two super minor comments.

[petejohanson]

Thanks!

[Katona]

Hi! Would it be possible to report the battery level of both halves in case of split keyboards? Either through the "master" (maybe with pressing some combo) or directly to the host via Bluetooth (if it's possible at all)?