Distinguish between simulation tools with defines

[JuanSapriza]

Structure changes

I modified the folders under sw/device/target. Now they are as follow

├── pynq-z2
│   └── x-heep.h
├── questasim
│   └── x-heep.h
├── vcs
│   └── x-heep.h
└── verilator
    └── x-heep.h

Inside each file, the new defines and messages are:

#pragma message ( "the x-heep.h for QUESTASIM is used" )
. . . 
#define UART_BAUDRATE       256000
#define TARGET_SIM          1
#define TARGET_QUESTASIM    1

This way conditional compilation can be achieved by differentiating between method (SIM/FPGA) or tool (VERILATOR, PYNQ_Z2,...)

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Application changes

Applications used to have a warning if the target was TARGET_SIM when the limitation only applied to verilator. Now I have added a forced

return EXIT_SUCCESS 

only if the target is TARGET_VERILATOR.

Additionally, the define TARGET_PYNQ_Z2 was changed for TARGET_FPGA except for those situations where the conditional was placed to decide on pynq-z2 specific details (like pins).

🙏 Please check if I have done this properly, as in some cases I was unsure.

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⚠️ IMPORTANT: Usage changes

Now, the make app command needs to take the simulation tool, not only TARGET=sim.
The default tool is verilator. Other options are questasim, vcs and pynq-z2.