Alexander Schermann / Gps generator - History

Added serial command to save startup frequency.

Added a serial command "S" to save current frequency as startup frequency.
If no startup frequency is found in EEPROM at boot time the default value of 1MHz will be used.
Introduced "fixstate" state machine to keep traxk of if fix is accuired or not, this makes the serial output quiter.
Added a "H" serial command to print Help text.

Merge pull request #3 from jepler/passthrough-and-runtime-frequency

[RFC] Passthrough and runtime frequency

Allow a jumper to enable booting directly to passthrough mode

Connect the SDA/SCL pins with a jumper or low value (e.g., 1K)
resistor to enable passthrough mode. Disconnect the jumper to return
to normal mode. These are pins 7/8 on the 10-pin connector, so they
are convenient to short with a standard .100 jumper block.

Support setting frequency via serial

To set the frequency, connect via serial and enter e.g.,:
F1M
followed by enter/return to set the frequency to 1000000Hz (1MHz)

Frequency can be set as a whole number, a whole number of kHz, or a
whole number of MHz. The commands are case insensitive. So all
the following set the frequency to 1MHz:
F1000000
F1000k
F1M

No validation is performed and nonsensical things can be requested,
so for example if you enter the nonsensical "F9MMM", you get
"GPS Initialized to output RF at 3800301568" This specific number
results from overflow of integer arithmetic and is also way out of
spec for the board, since the actual value requested is around 3.8GHz.