Add bool cast and improve charset options in settings. For #2095

Previously, tests could not be done on RadioSettingValueBoolean values.
This patch adds bool casting / test support so that checked boxes test True
and unchecked boxes test False.

RadioSettingValueString supported setting the charset, but only after
initializing the value. This fails if the value contains characters not in
the default charset. With this patch, the charset can be set during
initialization, so when the value is set, it is tested against the appropriate
charset.
Add support for Sainsonic AP510 APRS tracker. #2095

The Sainsonic AP510 is an integrated single-channel VHF radio and APRS tracker.
This patch adds support for configuring all of the parameters of the AP510 (one
channel and lots of settings).
Add test image for Sainsonic AP510 for #2095