refactoring of examples

[JuanSapriza]

During the testing of all example apps we faced several minor issues that can be tackled without affecting each other. We will be assessing those issues in this PR.
So far this includes:

• Changing occurrences of printf for PRINTF so it can be disabled by default for simulation and enabled for FPGA:

/* Change this value to 0 to disable prints for FPGA and enable them for simulation. */
#define DEFAULT_PRINTF_BEHAVIOR 1

/* By default, printfs are activated for FPGA and disabled for simulation. */
#ifdef TARGET_PYNQ_Z2 
    #define ENABLE_PRINTF DEFAULT_PRINTF_BEHAVIOR
#else 
    #define ENABLE_PRINTF !DEFAULT_PRINTF_BEHAVIOR
#endif

#if ENABLE_PRINTF
  #define PRINTF(fmt, ...)    printf(fmt, ## __VA_ARGS__)
#else
  #define PRINTF(...)
#endif 

• Add \r on PRINTFs to better visualize them on screen.
• Rename example apps so they follow the standard example_*.
• Reduced the size of large buffers in dma and i2s examples so they don't require more memory banks.
• Standardized some PRINTF messages of success and failure

[davideschiavone]

let me know when I need to review it, do not set it as a draft PR when it is ready @JuanSapriza

[JuanSapriza]

let me know when I need to review it, do not set it as a draft PR when it is ready @JuanSapriza

It should be good now. I was planning on leaving this as a draft until we finish all tests, but we can merge and open a new one if we need.

[davideschiavone]

we need to change the set_retentive app