[feature] OTP Read/Write

[mirkomatonti]

• I made serious effort to avoid creating duplicate or nearly similar issue
• Programmer/board type: [STLINK V2]
• Operating system an version: [Linux]
• stlink tools version and/or git commit hash: [ v1.7.0]
• stlink commandline tool name: [st-flash]
• Target chip (and board, if applicable): [STM32L4*]

Hello,

I need to read and write the otp area for a project of mine. I noticed that this feature has not being implemented yet so I did a quick implementation and it works. I know that OTP area is not standard across the STM32 line so how would that work in case I want to do a PR?
Can I just add support to L4x? (which is pretty easy as the OTP area is treated as FLASH) or do I need to do an implementation for all the supported boards?

My implementation is something along the line:

--area=otp read <output.bin> <len> //Read from the OTP start address to len
--area=otp write <input.bin> <add> //Write from a specific OTP address

To keep compatibility with the other boards I basically added two new fields (otp_base and otp_size) to the .chips file which in case they are not set the OTP read/write request returns a Not Implemented error. I suppose this will be helpful in case someone in the future wants to extend support to other boards.