Adds partial gl520m definitions and gv620mg definitions

[128keaton]

Hey, added a basic device definition for the GL520M and I believe this should fix compilation issues using modern GCC. I've not touched C in a while and these fixes were just based off my best guesses/intuition. I haven't noticed any unexpected behaviour but I'm only using the GL520M right now.

Should fix issue #20

PR updated to only include changes made to devices.yml for the definitions

[ckrey]

Do you have the protocol description for the GL520M available?
I guess the definition is not correct at the end where you skip numbers. Please check if “sent” timestamp is decoded correctly for all the different messages

[ckrey]

Thank you for your contribution.

• we decided to fix the compilation error by changing the function declaration rather than the function definition similar to what was done in the original mongoose repo

• you added compilation instructions and Makefile changes for Darwin are much appreciated, we will incorporate them

• changes to the device definitions we can only merge if you share the protocol description (e.g. GV520 @track Air Interface Protocol V1.01.pdf) with us via direct message.

[128keaton]

@ckrey understood, attached are the manuals for the GV620MG and the GL520M

GV620MG @Track Air Interface Protocol_V1.0D.pdf

GL520M Series @Track Air Interface Protocol_V2.02.pdf

(uploading here since they are available elsewhere for free, just hard to find)

[128keaton]

Here are the reports for the GL520M and the GV620MG

GL520M

• Position Related Report
  • GTFRI
  • GTGEO
  • GTRTL
  • GTPNL
  • GTNMR
• Device Power Report
  • GTPWR
• Device Information Report
  • GTINF
• Report for Querying
  • GTALC
  • GTCID
  • GTCSQ
  • GTVER
  • GTALS
• Event Report
  • GTPNA
  • GTPFA
  • GTBPL
  • GTTEM
  • GTUPC
  • GTLSA
  • GTLTA

GV620MG

• Position Related Report
  • GTTOW
  • GTEPS
  • GTDIS
  • GTIOB
  • GTFRI
  • GTSPD
  • GTSOS
  • GTGPJ
  • GTRTL
  • GTDOG
  • GTAIS
  • GTIGL
  • GTGES
  • GTERI
  • GTGIN
  • GTGOT
  • GTLBC
• Device Information Report
  • GTINF
• Report for Querying
  • GTGPT
  • GTALM
  • GTCID
  • GTCSQ
  • GTVER
  • GTBAT
  • GTIOS
  • GTTMZ
  • GTAIF
  • GTALS
  • GTALC
  • GTGSV
  • GTCVN
  • GTCSN
  • GTCML
• Event Report
  • GTPNA
  • GTPFA
  • GTMPN
  • GTMPF
  • GTBTC
  • GTSTC
  • GTBPL
  • GTSTT
  • GTIGN
  • GTIGF
  • GTIDN
  • GTIDF
  • GTGSM
  • GTGSS
  • GTSTR
  • GTSTP
  • GTLSP
  • GTFLA
  • GTDOS
  • GTTMP
  • GTPDP
  • GTUPC
  • GTCLT
  • GTCFU
  • GTBIE
• Transparent Data Transmission
  • GTDAT (short)
  • GTDAT (long)
• CANBUS Report Information
  • GTCAN